On Sun, Jul 10, 2016 at 02:12:24AM +0300, Laurent Pinchart wrote:
> Hi Sakari,
>
> On Sunday 10 Jul 2016 01:03:09 Sakari Ailus wrote:
> > On Sat, Jul 09, 2016 at 10:29:03PM +0300, Laurent Pinchart wrote:
> > > On Monday 09 May 2016 16:16:26 Sakari Ailus wrote:
> > >> Laurent Pinchart wrote:
> > >>> On Wednesday 04 May 2016 16:09:51 Sakari Ailus wrote:
> > >>>> Refactor copying the IOCTL argument structs from the user space and
> > >>>> back, in order to reduce code copied around and make the
> > >>>> implementation more robust.
> > >>>>
> > >>>> As a result, the copying is done while not holding the graph mutex.
> > >>>>
> > >>>> Signed-off-by: Sakari Ailus <sakari.ai...@linux.intel.com>
> > >>>> ---
> > >>>> since v2:
> > >>>>
> > >>>> - Remove function to calculate maximum argument size, replace by a
> > >>>> char array of 256 or kmalloc() if that's too small.
> > >>>>
> > >>>> drivers/media/media-device.c | 194 ++++++++++++++-------------------
> > >>>> 1 file changed, 94 insertions(+), 100 deletions(-)
> > >>>>
> > >>>> diff --git a/drivers/media/media-device.c
> > >>>> b/drivers/media/media-device.c
> > >>>> index 9b5a88d..0797e4b 100644
> > >>>> --- a/drivers/media/media-device.c
> > >>>> +++ b/drivers/media/media-device.c
> > >
> > > [snip]
> > >
> > >>>> @@ -453,10 +432,24 @@ static long __media_device_ioctl(
> > >>>>
> > >>>> info = &info_array[_IOC_NR(cmd)];
> > >>>>
> > >>>> + if (_IOC_SIZE(info->cmd) > sizeof(__karg)) {
> > >>>> + karg = kmalloc(_IOC_SIZE(info->cmd), GFP_KERNEL);
> > >>>> + if (!karg)
> > >>>> + return -ENOMEM;
> > >>>> + }
> > >>>> +
> > >>>> + info->arg_from_user(karg, arg, cmd);
> > >>>> +
> > >>>> mutex_lock(&dev->graph_mutex);
> > >>>> - ret = info->fn(dev, arg);
> > >>>> + ret = info->fn(dev, karg);
> > >>>> mutex_unlock(&dev->graph_mutex);
> > >>>>
> > >>>> + if (!ret)
> > >>>
> > >>> How about if (!ret && info->arg_to_user) instead, and getting rid of
> > >>> copy_arg_to_user_nop() ?
> > >>
> > >> I thought of that, but I decided to optimise the common case --- which
> > >> is that the argument is copied back and forth. Not copying the argument
> > >> back is a very special case, we use it for a single compat IOCTL.
> > >>
> > >> That said, we could use it for the proper ENUM_LINKS as well. Still that
> > >> does not change what's normal.
> > >
> > > We're talking about one comparison and one branching instruction (that
> > > will not be taken in the common case). Is that micro-optimization really
> > > worth it in an ioctl path that is not that performance-critical ? If you
> > > think it is, could you analyse what the impact of the
> > > copy_arg_to_user_nop() function on cache locality is for the common case ?
> > > ;-)
> >
> > I sense a certain amount of insistence in your arguments. Fine, I'll change
> > it.
>
> Thanks. I'll change that in the next version of the request API patches I
> will
> send out.
I think we rather should try to decrease the size of the set and get the
preparation patches in first.
I'm ready to send a pull request on these (after testing the rebased
patches), but it's been pending on the minimum arg size vs. list of
supported sizes discussion.
--
Sakari Ailus
e-mail: sakari.ai...@iki.fi XMPP: sai...@retiisi.org.uk
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