Re: [systemd-devel] libudev: subdirectories in sysfs (what does "available" mean?)

[Anne Mulhern]

- Original Message -
> From: "Greg KH" <gre...@linuxfoundation.org>
> To: "Anne Mulhern" <amulh...@redhat.com>
> Cc: "David Herrmann" <dh.herrm...@gmail.com>, "systemd" 
> <systemd-devel@lists.freedesktop.org>
> Sent: Tuesday, November 24, 2015 3:15:21 PM
> Subject: Re: [systemd-devel] libudev: subdirectories in sysfs (what does 
> "available" mean?)
> 
> On Tue, Nov 24, 2015 at 12:37:46PM -0500, Anne Mulhern wrote:
> > 
> > - Original Message -
> > > From: "Greg KH" <gre...@linuxfoundation.org>
> > > To: "Anne Mulhern" <amulh...@redhat.com>
> > > Cc: "David Herrmann" <dh.herrm...@gmail.com>, "systemd"
> > > <systemd-devel@lists.freedesktop.org>
> > > Sent: Tuesday, November 24, 2015 11:42:21 AM
> > > Subject: Re: [systemd-devel] libudev: subdirectories in sysfs (what does
> > > "available" mean?)
> > > 
> > > On Tue, Nov 24, 2015 at 10:37:08AM -0500, Anne Mulhern wrote:
> > > > 
> > > > 
> > > > 
> > > > 
> > > > ----- Original Message -
> > > > > From: "David Herrmann" <dh.herrm...@gmail.com>
> > > > > To: "Anne Mulhern" <amulh...@redhat.com>
> > > > > Cc: "systemd" <systemd-devel@lists.freedesktop.org>
> > > > > Sent: Tuesday, November 24, 2015 10:15:05 AM
> > > > > Subject: Re: [systemd-devel] libudev: subdirectories in sysfs (what
> > > > > does
> > > > > "available" mean?)
> > > > > 
> > > > > Hi
> > > > > 
> > > > > On Tue, Nov 24, 2015 at 3:54 PM, Anne Mulhern <amulh...@redhat.com>
> > > > > wrote:
> > > > > >> From: "David Herrmann" <dh.herrm...@gmail.com>
> > > > > >> On Tue, Nov 17, 2015 at 11:57 PM, Anne Mulhern
> > > > > >> <amulh...@redhat.com>
> > > > > >> wrote:
> > > > > >> >> From: "David Herrmann" <dh.herrm...@gmail.com>
> > > > > >> >> On Mon, Nov 16, 2015 at 5:35 PM, Anne Mulhern
> > > > > >> >> <amulh...@redhat.com>
> > > > > >> >> wrote:
> > > > > >> >> > libudev has some cooperating procedures that return the keys
> > > > > >> >> > for
> > > > > >> >> > a
> > > > > >> >> > bunch
> > > > > >> >> > of
> > > > > >> >> > sysfs attributes for a given device.
> > > > > >> >> >
> > > > > >> >> > These attributes all correspond to files that are stored in
> > > > > >> >> > the
> > > > > >> >> > sysfs
> > > > > >> >> > device directory.
> > > > > >> >> >
> > > > > >> >> > In the same directory there are sometimes subdirectories,
> > > > > >> >> > that
> > > > > >> >> > themselves
> > > > > >> >> > contain files
> > > > > >> >> > with information about their corresponding attribute. The dm
> > > > > >> >> > directory
> > > > > >> >> > is
> > > > > >> >> > one obvious
> > > > > >> >> > example.
> > > > > >> >> >
> > > > > >> >> > Are their any plans for libudev to add an ability to get the
> > > > > >> >> > values
> > > > > >> >> > from
> > > > > >> >> > these subdirectories
> > > > > >> >> > as some kind of attributes?
> > > > > >> >> >
> > > > > >> >> > If no, why?
> > > > > >> >>
> > > > > >> >> sd_device_get_sysattr_value(device, "foo/bar/baz", );
> > > > > >> >>
> > > > > >> >> This should work fine (or its udev_device_* equivalent).
> > > > > >> >>
> > > > > >> >> Btw., I recommend just using readdir(), open(), read(), and
> > > 

Re: [systemd-devel] libudev: subdirectories in sysfs (what does "available" mean?)

[Greg KH]

On Tue, Nov 24, 2015 at 12:37:46PM -0500, Anne Mulhern wrote:
> 
> - Original Message -
> > From: "Greg KH" <gre...@linuxfoundation.org>
> > To: "Anne Mulhern" <amulh...@redhat.com>
> > Cc: "David Herrmann" <dh.herrm...@gmail.com>, "systemd" 
> > <systemd-devel@lists.freedesktop.org>
> > Sent: Tuesday, November 24, 2015 11:42:21 AM
> > Subject: Re: [systemd-devel] libudev: subdirectories in sysfs (what does 
> > "available" mean?)
> > 
> > On Tue, Nov 24, 2015 at 10:37:08AM -0500, Anne Mulhern wrote:
> > > 
> > > 
> > > 
> > > 
> > > - Original Message -
> > > > From: "David Herrmann" <dh.herrm...@gmail.com>
> > > > To: "Anne Mulhern" <amulh...@redhat.com>
> > > > Cc: "systemd" <systemd-devel@lists.freedesktop.org>
> > > > Sent: Tuesday, November 24, 2015 10:15:05 AM
> > > > Subject: Re: [systemd-devel] libudev: subdirectories in sysfs (what does
> > > > "available" mean?)
> > > > 
> > > > Hi
> > > > 
> > > > On Tue, Nov 24, 2015 at 3:54 PM, Anne Mulhern <amulh...@redhat.com>
> > > > wrote:
> > > > >> From: "David Herrmann" <dh.herrm...@gmail.com>
> > > > >> On Tue, Nov 17, 2015 at 11:57 PM, Anne Mulhern <amulh...@redhat.com>
> > > > >> wrote:
> > > > >> >> From: "David Herrmann" <dh.herrm...@gmail.com>
> > > > >> >> On Mon, Nov 16, 2015 at 5:35 PM, Anne Mulhern 
> > > > >> >> <amulh...@redhat.com>
> > > > >> >> wrote:
> > > > >> >> > libudev has some cooperating procedures that return the keys for
> > > > >> >> > a
> > > > >> >> > bunch
> > > > >> >> > of
> > > > >> >> > sysfs attributes for a given device.
> > > > >> >> >
> > > > >> >> > These attributes all correspond to files that are stored in the
> > > > >> >> > sysfs
> > > > >> >> > device directory.
> > > > >> >> >
> > > > >> >> > In the same directory there are sometimes subdirectories, that
> > > > >> >> > themselves
> > > > >> >> > contain files
> > > > >> >> > with information about their corresponding attribute. The dm
> > > > >> >> > directory
> > > > >> >> > is
> > > > >> >> > one obvious
> > > > >> >> > example.
> > > > >> >> >
> > > > >> >> > Are their any plans for libudev to add an ability to get the
> > > > >> >> > values
> > > > >> >> > from
> > > > >> >> > these subdirectories
> > > > >> >> > as some kind of attributes?
> > > > >> >> >
> > > > >> >> > If no, why?
> > > > >> >>
> > > > >> >> sd_device_get_sysattr_value(device, "foo/bar/baz", );
> > > > >> >>
> > > > >> >> This should work fine (or its udev_device_* equivalent).
> > > > >> >>
> > > > >> >> Btw., I recommend just using readdir(), open(), read(), and
> > > > >> >> write().
> > > > >> >> sysfs is a filesystem, no reason to wrap all those commands.
> > > > >> >
> > > > >> > Thanks, I'm asking this more as the pyudev maintainer than as
> > > > >> > someone
> > > > >> > who actually wants these values.
> > > > >> >
> > > > >> > The funny thing is, I recently found out that the list obtained by
> > > > >> > udev_device_get_sysattr_list_entry () and friends contains so
> > > > >> > called "available" keys, but when those get passed to
> > > > >> > udev_device_get_sysattr_value () the result might be NULL.
> > > > >> > That makes sense in the sense that they might represent files
> > > > >> > that are unreadable.
> > > > >> >
> > > > >> > 

Re: [systemd-devel] libudev: subdirectories in sysfs (what does "available" mean?)

[Anne Mulhern]

- Original Message -
> From: "Greg KH" <gre...@linuxfoundation.org>
> To: "Anne Mulhern" <amulh...@redhat.com>
> Cc: "David Herrmann" <dh.herrm...@gmail.com>, "systemd" 
> <systemd-devel@lists.freedesktop.org>
> Sent: Tuesday, November 24, 2015 11:42:21 AM
> Subject: Re: [systemd-devel] libudev: subdirectories in sysfs (what does 
> "available" mean?)
> 
> On Tue, Nov 24, 2015 at 10:37:08AM -0500, Anne Mulhern wrote:
> > 
> > 
> > 
> > 
> > - Original Message -
> > > From: "David Herrmann" <dh.herrm...@gmail.com>
> > > To: "Anne Mulhern" <amulh...@redhat.com>
> > > Cc: "systemd" <systemd-devel@lists.freedesktop.org>
> > > Sent: Tuesday, November 24, 2015 10:15:05 AM
> > > Subject: Re: [systemd-devel] libudev: subdirectories in sysfs (what does
> > > "available" mean?)
> > > 
> > > Hi
> > > 
> > > On Tue, Nov 24, 2015 at 3:54 PM, Anne Mulhern <amulh...@redhat.com>
> > > wrote:
> > > >> From: "David Herrmann" <dh.herrm...@gmail.com>
> > > >> On Tue, Nov 17, 2015 at 11:57 PM, Anne Mulhern <amulh...@redhat.com>
> > > >> wrote:
> > > >> >> From: "David Herrmann" <dh.herrm...@gmail.com>
> > > >> >> On Mon, Nov 16, 2015 at 5:35 PM, Anne Mulhern <amulh...@redhat.com>
> > > >> >> wrote:
> > > >> >> > libudev has some cooperating procedures that return the keys for
> > > >> >> > a
> > > >> >> > bunch
> > > >> >> > of
> > > >> >> > sysfs attributes for a given device.
> > > >> >> >
> > > >> >> > These attributes all correspond to files that are stored in the
> > > >> >> > sysfs
> > > >> >> > device directory.
> > > >> >> >
> > > >> >> > In the same directory there are sometimes subdirectories, that
> > > >> >> > themselves
> > > >> >> > contain files
> > > >> >> > with information about their corresponding attribute. The dm
> > > >> >> > directory
> > > >> >> > is
> > > >> >> > one obvious
> > > >> >> > example.
> > > >> >> >
> > > >> >> > Are their any plans for libudev to add an ability to get the
> > > >> >> > values
> > > >> >> > from
> > > >> >> > these subdirectories
> > > >> >> > as some kind of attributes?
> > > >> >> >
> > > >> >> > If no, why?
> > > >> >>
> > > >> >> sd_device_get_sysattr_value(device, "foo/bar/baz", );
> > > >> >>
> > > >> >> This should work fine (or its udev_device_* equivalent).
> > > >> >>
> > > >> >> Btw., I recommend just using readdir(), open(), read(), and
> > > >> >> write().
> > > >> >> sysfs is a filesystem, no reason to wrap all those commands.
> > > >> >
> > > >> > Thanks, I'm asking this more as the pyudev maintainer than as
> > > >> > someone
> > > >> > who actually wants these values.
> > > >> >
> > > >> > The funny thing is, I recently found out that the list obtained by
> > > >> > udev_device_get_sysattr_list_entry () and friends contains so
> > > >> > called "available" keys, but when those get passed to
> > > >> > udev_device_get_sysattr_value () the result might be NULL.
> > > >> > That makes sense in the sense that they might represent files
> > > >> > that are unreadable.
> > > >> >
> > > >> > Now I find out that I can make up keys not in the results of
> > > >> > udev_device_get_sysattr_list_entry () and pass those to
> > > >> > udev_device_get_sysattr_value() and get a non-null result.
> > > >> >
> > > >> > So, what does "available" mean? Do these sysattr_list_entry()
> > > >> > methods give any useful information?
> > > >>
> > > >> "available" p

Re: [systemd-devel] libudev: subdirectories in sysfs (what does "available" mean?)

[Greg KH]

On Tue, Nov 24, 2015 at 10:37:08AM -0500, Anne Mulhern wrote:
> 
> 
> 
> 
> - Original Message -
> > From: "David Herrmann" <dh.herrm...@gmail.com>
> > To: "Anne Mulhern" <amulh...@redhat.com>
> > Cc: "systemd" <systemd-devel@lists.freedesktop.org>
> > Sent: Tuesday, November 24, 2015 10:15:05 AM
> > Subject: Re: [systemd-devel] libudev: subdirectories in sysfs (what does 
> > "available" mean?)
> > 
> > Hi
> > 
> > On Tue, Nov 24, 2015 at 3:54 PM, Anne Mulhern <amulh...@redhat.com> wrote:
> > >> From: "David Herrmann" <dh.herrm...@gmail.com>
> > >> On Tue, Nov 17, 2015 at 11:57 PM, Anne Mulhern <amulh...@redhat.com>
> > >> wrote:
> > >> >> From: "David Herrmann" <dh.herrm...@gmail.com>
> > >> >> On Mon, Nov 16, 2015 at 5:35 PM, Anne Mulhern <amulh...@redhat.com>
> > >> >> wrote:
> > >> >> > libudev has some cooperating procedures that return the keys for a
> > >> >> > bunch
> > >> >> > of
> > >> >> > sysfs attributes for a given device.
> > >> >> >
> > >> >> > These attributes all correspond to files that are stored in the 
> > >> >> > sysfs
> > >> >> > device directory.
> > >> >> >
> > >> >> > In the same directory there are sometimes subdirectories, that
> > >> >> > themselves
> > >> >> > contain files
> > >> >> > with information about their corresponding attribute. The dm
> > >> >> > directory
> > >> >> > is
> > >> >> > one obvious
> > >> >> > example.
> > >> >> >
> > >> >> > Are their any plans for libudev to add an ability to get the values
> > >> >> > from
> > >> >> > these subdirectories
> > >> >> > as some kind of attributes?
> > >> >> >
> > >> >> > If no, why?
> > >> >>
> > >> >> sd_device_get_sysattr_value(device, "foo/bar/baz", );
> > >> >>
> > >> >> This should work fine (or its udev_device_* equivalent).
> > >> >>
> > >> >> Btw., I recommend just using readdir(), open(), read(), and write().
> > >> >> sysfs is a filesystem, no reason to wrap all those commands.
> > >> >
> > >> > Thanks, I'm asking this more as the pyudev maintainer than as someone
> > >> > who actually wants these values.
> > >> >
> > >> > The funny thing is, I recently found out that the list obtained by
> > >> > udev_device_get_sysattr_list_entry () and friends contains so
> > >> > called "available" keys, but when those get passed to
> > >> > udev_device_get_sysattr_value () the result might be NULL.
> > >> > That makes sense in the sense that they might represent files
> > >> > that are unreadable.
> > >> >
> > >> > Now I find out that I can make up keys not in the results of
> > >> > udev_device_get_sysattr_list_entry () and pass those to
> > >> > udev_device_get_sysattr_value() and get a non-null result.
> > >> >
> > >> > So, what does "available" mean? Do these sysattr_list_entry()
> > >> > methods give any useful information?
> > >>
> > >> "available" probably means attributes which are direct descendants of
> > >> the device. That is, sysattr_list_entry() only lists such direct
> > >> descendants, while sysattr_value() allows you to query anything (you
> > >> probably can even pass "foo/../../bar/baz").
> > >>
> > >> Thanks
> > >> David
> > >>
> > >
> > > I think it's yet more complicated than that. For example,
> > > 'dm' (for device mapper) is not in the list of available
> > > attributes, but 'dm/name' is certainly an attribute that
> > > can be read by sysattr_value().
> > 
> > 'dm' is not an attribute, so it will never be listed as available
> > attribute. Directories are never treated as attributes.
> > 
> > Thanks
> > David
> > 
> 
> But "bdi" is listed as an attribute, and is a directory.

Can you provide a "full" path here that you are looking at (or the
output of 'tree' or 'find' on the device), so that I can try to figure
this out?

Odds are, dm is doing something "odd"...

thanks,

greg k-h
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Re: [systemd-devel] libudev: subdirectories in sysfs (what does "available" mean?)

[David Herrmann]

Hi

On Tue, Nov 17, 2015 at 11:57 PM, Anne Mulhern  wrote:
>> From: "David Herrmann" 
>> On Mon, Nov 16, 2015 at 5:35 PM, Anne Mulhern  wrote:
>> > libudev has some cooperating procedures that return the keys for a bunch of
>> > sysfs attributes for a given device.
>> >
>> > These attributes all correspond to files that are stored in the sysfs
>> > device directory.
>> >
>> > In the same directory there are sometimes subdirectories, that themselves
>> > contain files
>> > with information about their corresponding attribute. The dm directory is
>> > one obvious
>> > example.
>> >
>> > Are their any plans for libudev to add an ability to get the values from
>> > these subdirectories
>> > as some kind of attributes?
>> >
>> > If no, why?
>>
>> sd_device_get_sysattr_value(device, "foo/bar/baz", );
>>
>> This should work fine (or its udev_device_* equivalent).
>>
>> Btw., I recommend just using readdir(), open(), read(), and write().
>> sysfs is a filesystem, no reason to wrap all those commands.
>
> Thanks, I'm asking this more as the pyudev maintainer than as someone
> who actually wants these values.
>
> The funny thing is, I recently found out that the list obtained by
> udev_device_get_sysattr_list_entry () and friends contains so
> called "available" keys, but when those get passed to
> udev_device_get_sysattr_value () the result might be NULL.
> That makes sense in the sense that they might represent files
> that are unreadable.
>
> Now I find out that I can make up keys not in the results of
> udev_device_get_sysattr_list_entry () and pass those to
> udev_device_get_sysattr_value() and get a non-null result.
>
> So, what does "available" mean? Do these sysattr_list_entry()
> methods give any useful information?

"available" probably means attributes which are direct descendants of
the device. That is, sysattr_list_entry() only lists such direct
descendants, while sysattr_value() allows you to query anything (you
probably can even pass "foo/../../bar/baz").

Thanks
David
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Re: [systemd-devel] libudev: subdirectories in sysfs (what does "available" mean?)

[Anne Mulhern]

- Original Message -
> From: "David Herrmann" <dh.herrm...@gmail.com>
> To: "Anne Mulhern" <amulh...@redhat.com>
> Cc: "systemd" <systemd-devel@lists.freedesktop.org>
> Sent: Tuesday, November 24, 2015 4:43:24 AM
> Subject: Re: [systemd-devel] libudev: subdirectories in sysfs (what does 
> "available" mean?)
> 
> Hi
> 
> On Tue, Nov 17, 2015 at 11:57 PM, Anne Mulhern <amulh...@redhat.com> wrote:
> >> From: "David Herrmann" <dh.herrm...@gmail.com>
> >> On Mon, Nov 16, 2015 at 5:35 PM, Anne Mulhern <amulh...@redhat.com> wrote:
> >> > libudev has some cooperating procedures that return the keys for a bunch
> >> > of
> >> > sysfs attributes for a given device.
> >> >
> >> > These attributes all correspond to files that are stored in the sysfs
> >> > device directory.
> >> >
> >> > In the same directory there are sometimes subdirectories, that
> >> > themselves
> >> > contain files
> >> > with information about their corresponding attribute. The dm directory
> >> > is
> >> > one obvious
> >> > example.
> >> >
> >> > Are their any plans for libudev to add an ability to get the values from
> >> > these subdirectories
> >> > as some kind of attributes?
> >> >
> >> > If no, why?
> >>
> >> sd_device_get_sysattr_value(device, "foo/bar/baz", );
> >>
> >> This should work fine (or its udev_device_* equivalent).
> >>
> >> Btw., I recommend just using readdir(), open(), read(), and write().
> >> sysfs is a filesystem, no reason to wrap all those commands.
> >
> > Thanks, I'm asking this more as the pyudev maintainer than as someone
> > who actually wants these values.
> >
> > The funny thing is, I recently found out that the list obtained by
> > udev_device_get_sysattr_list_entry () and friends contains so
> > called "available" keys, but when those get passed to
> > udev_device_get_sysattr_value () the result might be NULL.
> > That makes sense in the sense that they might represent files
> > that are unreadable.
> >
> > Now I find out that I can make up keys not in the results of
> > udev_device_get_sysattr_list_entry () and pass those to
> > udev_device_get_sysattr_value() and get a non-null result.
> >
> > So, what does "available" mean? Do these sysattr_list_entry()
> > methods give any useful information?
> 
> "available" probably means attributes which are direct descendants of
> the device. That is, sysattr_list_entry() only lists such direct
> descendants, while sysattr_value() allows you to query anything (you
> probably can even pass "foo/../../bar/baz").
> 
> Thanks
> David
> 

I think it's yet more complicated than that. For example,
'dm' (for device mapper) is not in the list of available
attributes, but 'dm/name' is certainly an attribute that
can be read by sysattr_value().

- mulhern
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Re: [systemd-devel] libudev: subdirectories in sysfs (what does "available" mean?)

[David Herrmann]

Hi

On Tue, Nov 24, 2015 at 3:54 PM, Anne Mulhern  wrote:
>> From: "David Herrmann" 
>> On Tue, Nov 17, 2015 at 11:57 PM, Anne Mulhern  wrote:
>> >> From: "David Herrmann" 
>> >> On Mon, Nov 16, 2015 at 5:35 PM, Anne Mulhern  wrote:
>> >> > libudev has some cooperating procedures that return the keys for a bunch
>> >> > of
>> >> > sysfs attributes for a given device.
>> >> >
>> >> > These attributes all correspond to files that are stored in the sysfs
>> >> > device directory.
>> >> >
>> >> > In the same directory there are sometimes subdirectories, that
>> >> > themselves
>> >> > contain files
>> >> > with information about their corresponding attribute. The dm directory
>> >> > is
>> >> > one obvious
>> >> > example.
>> >> >
>> >> > Are their any plans for libudev to add an ability to get the values from
>> >> > these subdirectories
>> >> > as some kind of attributes?
>> >> >
>> >> > If no, why?
>> >>
>> >> sd_device_get_sysattr_value(device, "foo/bar/baz", );
>> >>
>> >> This should work fine (or its udev_device_* equivalent).
>> >>
>> >> Btw., I recommend just using readdir(), open(), read(), and write().
>> >> sysfs is a filesystem, no reason to wrap all those commands.
>> >
>> > Thanks, I'm asking this more as the pyudev maintainer than as someone
>> > who actually wants these values.
>> >
>> > The funny thing is, I recently found out that the list obtained by
>> > udev_device_get_sysattr_list_entry () and friends contains so
>> > called "available" keys, but when those get passed to
>> > udev_device_get_sysattr_value () the result might be NULL.
>> > That makes sense in the sense that they might represent files
>> > that are unreadable.
>> >
>> > Now I find out that I can make up keys not in the results of
>> > udev_device_get_sysattr_list_entry () and pass those to
>> > udev_device_get_sysattr_value() and get a non-null result.
>> >
>> > So, what does "available" mean? Do these sysattr_list_entry()
>> > methods give any useful information?
>>
>> "available" probably means attributes which are direct descendants of
>> the device. That is, sysattr_list_entry() only lists such direct
>> descendants, while sysattr_value() allows you to query anything (you
>> probably can even pass "foo/../../bar/baz").
>>
>> Thanks
>> David
>>
>
> I think it's yet more complicated than that. For example,
> 'dm' (for device mapper) is not in the list of available
> attributes, but 'dm/name' is certainly an attribute that
> can be read by sysattr_value().

'dm' is not an attribute, so it will never be listed as available
attribute. Directories are never treated as attributes.

Thanks
David
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Re: [systemd-devel] libudev: subdirectories in sysfs (what does "available" mean?)

[Anne Mulhern]

- Original Message -
> From: "David Herrmann" <dh.herrm...@gmail.com>
> To: "Anne Mulhern" <amulh...@redhat.com>
> Cc: "systemd" <systemd-devel@lists.freedesktop.org>
> Sent: Tuesday, November 24, 2015 10:15:05 AM
> Subject: Re: [systemd-devel] libudev: subdirectories in sysfs (what does 
> "available" mean?)
> 
> Hi
> 
> On Tue, Nov 24, 2015 at 3:54 PM, Anne Mulhern <amulh...@redhat.com> wrote:
> >> From: "David Herrmann" <dh.herrm...@gmail.com>
> >> On Tue, Nov 17, 2015 at 11:57 PM, Anne Mulhern <amulh...@redhat.com>
> >> wrote:
> >> >> From: "David Herrmann" <dh.herrm...@gmail.com>
> >> >> On Mon, Nov 16, 2015 at 5:35 PM, Anne Mulhern <amulh...@redhat.com>
> >> >> wrote:
> >> >> > libudev has some cooperating procedures that return the keys for a
> >> >> > bunch
> >> >> > of
> >> >> > sysfs attributes for a given device.
> >> >> >
> >> >> > These attributes all correspond to files that are stored in the sysfs
> >> >> > device directory.
> >> >> >
> >> >> > In the same directory there are sometimes subdirectories, that
> >> >> > themselves
> >> >> > contain files
> >> >> > with information about their corresponding attribute. The dm
> >> >> > directory
> >> >> > is
> >> >> > one obvious
> >> >> > example.
> >> >> >
> >> >> > Are their any plans for libudev to add an ability to get the values
> >> >> > from
> >> >> > these subdirectories
> >> >> > as some kind of attributes?
> >> >> >
> >> >> > If no, why?
> >> >>
> >> >> sd_device_get_sysattr_value(device, "foo/bar/baz", );
> >> >>
> >> >> This should work fine (or its udev_device_* equivalent).
> >> >>
> >> >> Btw., I recommend just using readdir(), open(), read(), and write().
> >> >> sysfs is a filesystem, no reason to wrap all those commands.
> >> >
> >> > Thanks, I'm asking this more as the pyudev maintainer than as someone
> >> > who actually wants these values.
> >> >
> >> > The funny thing is, I recently found out that the list obtained by
> >> > udev_device_get_sysattr_list_entry () and friends contains so
> >> > called "available" keys, but when those get passed to
> >> > udev_device_get_sysattr_value () the result might be NULL.
> >> > That makes sense in the sense that they might represent files
> >> > that are unreadable.
> >> >
> >> > Now I find out that I can make up keys not in the results of
> >> > udev_device_get_sysattr_list_entry () and pass those to
> >> > udev_device_get_sysattr_value() and get a non-null result.
> >> >
> >> > So, what does "available" mean? Do these sysattr_list_entry()
> >> > methods give any useful information?
> >>
> >> "available" probably means attributes which are direct descendants of
> >> the device. That is, sysattr_list_entry() only lists such direct
> >> descendants, while sysattr_value() allows you to query anything (you
> >> probably can even pass "foo/../../bar/baz").
> >>
> >> Thanks
> >> David
> >>
> >
> > I think it's yet more complicated than that. For example,
> > 'dm' (for device mapper) is not in the list of available
> > attributes, but 'dm/name' is certainly an attribute that
> > can be read by sysattr_value().
> 
> 'dm' is not an attribute, so it will never be listed as available
> attribute. Directories are never treated as attributes.
> 
> Thanks
> David
> 

But "bdi" is listed as an attribute, and is a directory.

- mulhern
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Re: [systemd-devel] libudev: subdirectories in sysfs (what does "available" mean?)

[David Herrmann]

Hi

On Tue, Nov 24, 2015 at 4:37 PM, Anne Mulhern  wrote:
>> From: "David Herrmann" 
>> On Tue, Nov 24, 2015 at 3:54 PM, Anne Mulhern  wrote:
>> >> From: "David Herrmann" 
>> >> On Tue, Nov 17, 2015 at 11:57 PM, Anne Mulhern 
>> >> wrote:
>> >> >> From: "David Herrmann" 
>> >> >> On Mon, Nov 16, 2015 at 5:35 PM, Anne Mulhern 
>> >> >> wrote:
>> >> >> > libudev has some cooperating procedures that return the keys for a
>> >> >> > bunch
>> >> >> > of
>> >> >> > sysfs attributes for a given device.
>> >> >> >
>> >> >> > These attributes all correspond to files that are stored in the sysfs
>> >> >> > device directory.
>> >> >> >
>> >> >> > In the same directory there are sometimes subdirectories, that
>> >> >> > themselves
>> >> >> > contain files
>> >> >> > with information about their corresponding attribute. The dm
>> >> >> > directory
>> >> >> > is
>> >> >> > one obvious
>> >> >> > example.
>> >> >> >
>> >> >> > Are their any plans for libudev to add an ability to get the values
>> >> >> > from
>> >> >> > these subdirectories
>> >> >> > as some kind of attributes?
>> >> >> >
>> >> >> > If no, why?
>> >> >>
>> >> >> sd_device_get_sysattr_value(device, "foo/bar/baz", );
>> >> >>
>> >> >> This should work fine (or its udev_device_* equivalent).
>> >> >>
>> >> >> Btw., I recommend just using readdir(), open(), read(), and write().
>> >> >> sysfs is a filesystem, no reason to wrap all those commands.
>> >> >
>> >> > Thanks, I'm asking this more as the pyudev maintainer than as someone
>> >> > who actually wants these values.
>> >> >
>> >> > The funny thing is, I recently found out that the list obtained by
>> >> > udev_device_get_sysattr_list_entry () and friends contains so
>> >> > called "available" keys, but when those get passed to
>> >> > udev_device_get_sysattr_value () the result might be NULL.
>> >> > That makes sense in the sense that they might represent files
>> >> > that are unreadable.
>> >> >
>> >> > Now I find out that I can make up keys not in the results of
>> >> > udev_device_get_sysattr_list_entry () and pass those to
>> >> > udev_device_get_sysattr_value() and get a non-null result.
>> >> >
>> >> > So, what does "available" mean? Do these sysattr_list_entry()
>> >> > methods give any useful information?
>> >>
>> >> "available" probably means attributes which are direct descendants of
>> >> the device. That is, sysattr_list_entry() only lists such direct
>> >> descendants, while sysattr_value() allows you to query anything (you
>> >> probably can even pass "foo/../../bar/baz").
>> >>
>> >> Thanks
>> >> David
>> >>
>> >
>> > I think it's yet more complicated than that. For example,
>> > 'dm' (for device mapper) is not in the list of available
>> > attributes, but 'dm/name' is certainly an attribute that
>> > can be read by sysattr_value().
>>
>> 'dm' is not an attribute, so it will never be listed as available
>> attribute. Directories are never treated as attributes.
>>
>> Thanks
>> David
>>
>
> But "bdi" is listed as an attribute, and is a directory.

Weird. Tom, any comments?
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Re: [systemd-devel] libudev: subdirectories in sysfs (what does "available" mean?)

[Anne Mulhern]

- Original Message -
> From: "David Herrmann" 
> To: "Anne Mulhern" 
> Cc: "systemd" 
> Sent: Tuesday, November 17, 2015 10:48:19 AM
> Subject: Re: [systemd-devel] libudev: subdirectories in sysfs
> 
> Hi
> 
> On Mon, Nov 16, 2015 at 5:35 PM, Anne Mulhern  wrote:
> > Hi!
> >
> > libudev has some cooperating procedures that return the keys for a bunch of
> > sysfs attributes for a given device.
> >
> > These attributes all correspond to files that are stored in the sysfs
> > device directory.
> >
> > In the same directory there are sometimes subdirectories, that themselves
> > contain files
> > with information about their corresponding attribute. The dm directory is
> > one obvious
> > example.
> >
> > Are their any plans for libudev to add an ability to get the values from
> > these subdirectories
> > as some kind of attributes?
> >
> > If no, why?
> 
> sd_device_get_sysattr_value(device, "foo/bar/baz", );
> 
> This should work fine (or its udev_device_* equivalent).
> 
> Btw., I recommend just using readdir(), open(), read(), and write().
> sysfs is a filesystem, no reason to wrap all those commands.
> 
> Thanks
> David
> 

Thanks, I'm asking this more as the pyudev maintainer than as someone
who actually wants these values.

The funny thing is, I recently found out that the list obtained by
udev_device_get_sysattr_list_entry () and friends contains so
called "available" keys, but when those get passed to 
udev_device_get_sysattr_value () the result might be NULL.
That makes sense in the sense that they might represent files
that are unreadable. 

Now I find out that I can make up keys not in the results of
udev_device_get_sysattr_list_entry () and pass those to
udev_device_get_sysattr_value() and get a non-null result.

So, what does "available" mean? Do these sysattr_list_entry()
methods give any useful information?

- mulhern

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