On Sun, Mar 7, 2021 at 11:28 PM Marek Szyprowski <m.szyprow...@samsung.com> wrote: > > Hi Saravana, > > On 05.03.2021 19:02, Saravana Kannan wrote: > > On Fri, Mar 5, 2021 at 3:45 AM Marek Szyprowski > > <m.szyprow...@samsung.com> wrote: > >> On 04.03.2021 20:51, Saravana Kannan wrote: > >>> The uevents generated for an amba device need PID and CID information > >>> that's available only when the amba device is powered on, clocked and > >>> out of reset. So, if those resources aren't available, the information > >>> can't be read to generate the uevents. To workaround this requirement, > >>> if the resources weren't available, the device addition was deferred and > >>> retried periodically. > >>> > >>> However, this deferred addition retry isn't based on resources becoming > >>> available. Instead, it's retried every 5 seconds and causes arbitrary > >>> probe delays for amba devices and their consumers. > >>> > >>> Also, maintaining a separate deferred-probe like mechanism is > >>> maintenance headache. > >>> > >>> With this commit, instead of deferring the device addition, we simply > >>> defer the generation of uevents for the device and probing of the device > >>> (because drivers needs PID and CID to match) until the PID and CID > >>> information can be read. This allows us to delete all the amba specific > >>> deferring code and also avoid the arbitrary probing delays. > >>> > >>> Cc: Rob Herring <r...@kernel.org> > >>> Cc: Ulf Hansson <ulf.hans...@linaro.org> > >>> Cc: John Stultz <john.stu...@linaro.org> > >>> Cc: Saravana Kannan <sarava...@google.com> > >>> Cc: Linus Walleij <linus.wall...@linaro.org> > >>> Cc: Sudeep Holla <sudeep.ho...@arm.com> > >>> Cc: Nicolas Saenz Julienne <nsaenzjulie...@suse.de> > >>> Cc: Geert Uytterhoeven <geert+rene...@glider.be> > >>> Cc: Marek Szyprowski <m.szyprow...@samsung.com> > >>> Cc: Russell King <li...@armlinux.org.uk> > >>> Signed-off-by: Saravana Kannan <sarava...@google.com> > >>> --- > >>> > >>> v1 -> v2: > >>> - Dropped RFC tag > >>> - Complete rewrite to not use stub devices. > >>> v2 -> v3: > >>> - Flipped the if() condition for hard-coded periphids. > >>> - Added a stub driver to handle the case where all amba drivers are > >>> modules loaded by uevents. > >>> - Cc Marek after I realized I forgot to add him. > >>> > >>> Marek, > >>> > >>> Would you mind testing this? It looks okay with my limited testing. > >> It looks it works fine on my test systems. I've checked current > >> linux-next and this patch. You can add: > >> > >> Tested-by: Marek Szyprowski <m.szyprow...@samsung.com> > > Hi Marek, > > > > Thanks! Does your test set up have amda drivers that are loaded based > > on uevents? That's the one I couldn't test. > > I've checked both, the built-in and all amba drivers compiled as > modules, loaded by udev. Both works fine here. > > >> I've briefly scanned the code and I'm curious how does it work. Does it > >> depend on the recently introduced "fw_devlink=on" feature? I don't see > >> other mechanism, which would trigger matching amba device if pm domains, > >> clocks or resets were not available on time to read pid/cid while adding > >> a device... > > No, it does not depend on fw_devlink or device links in any way. > > > > When a device is attempted to be probed (when it's added or during > > deferred probe), it's matched with all the drivers on the bus. > > When a new driver is registered to a bus, all devices in that bus are > > matched with the driver to see if they'll work together. > > That's how match is called. And match() can return -EPROBE_DEFER and > > that'll cause the device to be put in the deferred probe list by > > driver core. > > > > The tricky part in this patch was the uevent handling and the > > chicken-and-egg issue I talk about in the comments. > > Thanks for the explanation. This EPROBE_DEFER support in match() > callback must be something added after I crafted that periodic retry > based workaround. >
I think it got in just a few months before your patches, but your patches worked :) I actually don't like match returning -EPROBE_DEFER, but I can work around it for some of my fw_devlink optimization plans. More context here: https://lore.kernel.org/lkml/CAGETcx_qO4vxTSyBtBR2k7fd_3rGJF42iBbJH37HPNw=fhe...@mail.gmail.com/ -Saravana
