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. Best regards -- Marek Szyprowski, PhD Samsung R&D Institute Poland