Re: [PATCH v3] amba: Remove deferred device addition

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hi Saravana,

On 05.03.2021 19:02, Saravana Kannan wrote:
> On Fri, Mar 5, 2021 at 3:45 AM Marek Szyprowski
> <m.szyprowski@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 <robh@kernel.org>
>>> Cc: Ulf Hansson <ulf.hansson@linaro.org>
>>> Cc: John Stultz <john.stultz@linaro.org>
>>> Cc: Saravana Kannan <saravanak@google.com>
>>> Cc: Linus Walleij <linus.walleij@linaro.org>
>>> Cc: Sudeep Holla <sudeep.holla@arm.com>
>>> Cc: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
>>> Cc: Geert Uytterhoeven <geert+renesas@glider.be>
>>> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
>>> Cc: Russell King <linux@armlinux.org.uk>
>>> Signed-off-by: Saravana Kannan <saravanak@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.szyprowski@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