[PATCH] drm/edid/firmware: stop using throwaway platform device

[Matthieu CHARETTE]

Hi,

Can you tell me what are we waiting for? Maybe I can help.

Thanks.

Matthieu

On Wed, Oct 12 2022 at 07:16:29 PM +0200, Matthieu CHARETTE 
<matthieu.charette at gmail.com> wrote:
> By crash, I mean that an error is returned here: 
> https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux.git/+/refs/heads/master/drivers/gpu/drm/drm_edid_load.c#195
> I don't really know what happens next, but on my machine the built-in 
> screen and the external remains dark. Also the kernel seems to 
> freeze. I suspect a kernel panic, but I'm not sure. Anyway, the error 
> is definitely not well handled, and a fix would be great.
> Also, request_firmware() will crash if called for the first time on 
> the resume path because the file system isn't reachable on the resume 
> process. And no cache is available for this firmware. So I guess that 
> in this case, request_firmware() returns an error.
> Suspend-plug-resume case is not my priority nether as long as it 
> doesn't make the system crash (Which is currently the case).
> 
> On Wed, Oct 12 2022 at 11:25:59 AM +0300, Jani Nikula 
> <jani.nikula at intel.com> wrote:
>> On Tue, 11 Oct 2022, Matthieu CHARETTE <matthieu.charette at gmail.com> 
>> wrote:
>>>  Currently the EDID is requested during the resume. But since it's
>>>  requested too early, this means before the filesystem is mounted, 
>>> the
>>>  firmware request fails. This make the DRM driver crash when 
>>> resuming.
>>>  This kind of issue should be prevented by the firmware caching 
>>> process
>>>  which cache every firmware requested for the next resume. But 
>>> since we
>>>  are using a temporary device, the firmware isn't cached on suspend
>>>  since the device doesn't work anymore.
>>>  When using a non temporary device to get the EDID, the firmware 
>>> will
>>>  be cached on suspend for the next resume. So requesting the 
>>> firmware
>>>  during resume will succeed.
>>>  But if the firmware has never been requested since the boot, this
>>>  means that the monitor isn't plugged since the boot. The kernel 
>>> will
>>>  not be caching the EDID. So if we plug the monitor while the 
>>> machine
>>>  is suspended. The resume will fail to load the firmware. And the 
>>> DRM
>>>  driver will crash.
>>>  So basically, your fix should solve the issue except for the case
>>>  where the monitor hasn't been plugged since boot and is plugged 
>>> while
>>>  the machine is suspended.
>>>  I hope I was clear. Tell me if I wasn't. I'm not really good at 
>>> explaining.
>> 
>> That was a pretty good explanation. The only thing I'm missing is 
>> what
>> the failure mode is exactly when you claim the driver will crash. Why
>> would request_firmware() "crash" if called for the first time on the
>> resume path?
>> 
>> I'm not sure I care much about not being able to load the firmware 
>> EDID
>> in the suspend-plug-resume case (as this can be remedied with a
>> subsequent modeset), but obviously any errors need to be handled
>> gracefully, without crashing.
>> 
>> BR,
>> Jani.
>> 
>> 
>> --
>> Jani Nikula, Intel Open Source Graphics Center
> 
>