[PATCH] drm/client: Send hotplug event after registering a client

[Limonciello, Mario]

+regressions
On 7/10/2023 04:58, Thomas Zimmermann wrote:
> Hi
> 
> Am 10.07.23 um 11:52 schrieb Javier Martinez Canillas:
>> Thomas Zimmermann <tzimmermann at suse.de> writes:
>>
>> Hello Thomas,
>>
>>> Generate a hotplug event after registering a client to allow the
>>> client to configure its display. Remove the hotplug calls from the
>>> existing clients for fbdev emulation. This change fixes a concurrency
>>> bug between registering a client and receiving events from the DRM
>>> core. The bug is present in the fbdev emulation of all drivers.
>>>
>>> The fbdev emulation currently generates a hotplug event before
>>> registering the client to the device. For each new output, the DRM
>>> core sends an additional hotplug event to each registered client.
>>>
>>> If the DRM core detects first output between sending the artificial
>>> hotplug and registering the device, the output's hotplug event gets
>>> lost. If this is the first output, the fbdev console display remains
>>> dark. This has been observed with amdgpu and fbdev-generic.
>>>
>>> Fix this by adding hotplug generation directly to the client's
>>> register helper drm_client_register(). Registering the client and
>>> receiving events are serialized by struct drm_device.clientlist_mutex.
>>> So an output is either configured by the initial hotplug event, or
>>> the client has already been registered.
>>>
>>> The bug was originally added in commit 6e3f17ee73f7 ("drm/fb-helper:
>>> generic: Call drm_client_add() after setup is done"), in which adding
>>> a client and receiving a hotplug event switched order. It was hidden,
>>> as most hardware and drivers have at least on static output configured.
>>> Other drivers didn't use the internal DRM client or still had struct
>>> drm_mode_config_funcs.output_poll_changed set. That callback handled
>>> hotplug events as well. After not setting the callback in amdgpu in
>>> commit 0e3172bac3f4 ("drm/amdgpu: Don't set struct
>>> drm_driver.output_poll_changed"), amdgpu did not show a framebuffer
>>> console if output events got lost. The bug got copy-pasted from
>>> fbdev-generic into the other fbdev emulation.
>>>
>>> Reported-by: Moritz Duge <MoritzDuge at kolahilft.de>
>>> Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/2649
>>
>> Aren't you missing a Fixes: for 0e3172bac3f4 too? Since that's the commit
>> that unmasked the bug for amdgpu, IMO that is the most important to list.
> 
> Well, OK.
> 
>>
>>> Fixes: 6e3f17ee73f7 ("drm/fb-helper: generic: Call drm_client_add() 
>>> after setup is done")
>>> Fixes: 8ab59da26bc0 ("drm/fb-helper: Move generic fbdev emulation 
>>> into separate source file")
>>> Fixes: b79fe9abd58b ("drm/fbdev-dma: Implement fbdev emulation for 
>>> GEM DMA helpers")
>>> Fixes: 63c381552f69 ("drm/armada: Implement fbdev emulation as 
>>> in-kernel client")
>>> Fixes: 49953b70e7d3 ("drm/exynos: Implement fbdev emulation as 
>>> in-kernel client")
>>> Fixes: 8f1aaccb04b7 ("drm/gma500: Implement client-based fbdev 
>>> emulation")
>>> Fixes: 940b869c2f2f ("drm/msm: Implement fbdev emulation as in-kernel 
>>> client")
>>> Fixes: 9e69bcd88e45 ("drm/omapdrm: Implement fbdev emulation as 
>>> in-kernel client")
>>> Fixes: e317a69fe891 ("drm/radeon: Implement client-based fbdev 
>>> emulation")
>>> Fixes: 71ec16f45ef8 ("drm/tegra: Implement fbdev emulation as 
>>> in-kernel client")
>>> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
>>> Tested-by: Moritz Duge <MoritzDuge at kolahilft.de>
>>> Tested-by: Torsten Krah <krah.tm at gmail.com>
>>> Tested-by: Paul Schyska <pschyska at gmail.com>
>>> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
>>> Cc: David Airlie <airlied at gmail.com>
>>> Cc: Noralf Trønnes <noralf at tronnes.org>
>>> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
>>> Cc: Maxime Ripard <mripard at kernel.org>
>>> Cc: Javier Martinez Canillas <javierm at redhat.com>
>>> Cc: Russell King <linux at armlinux.org.uk>
>>> Cc: Inki Dae <inki.dae at samsung.com>
>>> Cc: Seung-Woo Kim <sw0312.kim at samsung.com>
>>> Cc: Kyungmin Park <kyungmin.park at samsung.com>
>>> Cc: Krzysztof Kozlowski <krzysztof.kozlowski at linaro.org>
>>> Cc: Patrik Jakobsson <patrik.r.jakobsson at gmail.com>
>>> Cc: Rob Clark <robdclark at gmail.com>
>>> Cc: Abhinav Kumar <quic_abhinavk at quicinc.com>
>>> Cc: Dmitry Baryshkov <dmitry.baryshkov at linaro.org>
>>> Cc: Tomi Valkeinen <tomi.valkeinen at ideasonboard.com>
>>> Cc: Alex Deucher <alexander.deucher at amd.com>
>>> Cc: "Christian König" <christian.koenig at amd.com>
>>> Cc: "Pan, Xinhui" <Xinhui.Pan at amd.com>
>>> Cc: Thierry Reding <thierry.reding at gmail.com>
>>> Cc: Mikko Perttunen <mperttunen at nvidia.com>
>>> Cc: dri-devel at lists.freedesktop.org
>>> Cc: linux-kernel at vger.kernel.org
>>> Cc: linux-arm-kernel at lists.infradead.org
>>> Cc: linux-samsung-soc at vger.kernel.org
>>> Cc: linux-arm-msm at vger.kernel.org
>>> Cc: freedreno at lists.freedesktop.org
>>> Cc: amd-gfx at lists.freedesktop.org
>>> Cc: linux-tegra at vger.kernel.org
>>> Cc: dri-devel at lists.freedesktop.org
>>> Cc: <stable at vger.kernel.org> # v5.2+
>>
>> While it's true that the but was introduced by commit 6e3f17ee73f7 and 
>> that
>> landed in v5.2, I wonder if this patch could even be applied to such 
>> olders
>> Linux versions. Probably in practice it would be at most backported to
>> v6.2, which is the release that exposed the bug for the amdgpu driver.
> 
> No idea. The fix looks simple enough, but a lot has changed in the 
> surrounding code.
> 

Actually it needs to go to at least 6.1.y.

Moritz found it in 6.1.35 (not present in 6.1.34).


> Best regards
> Thomas
> 
>>
>> Your explanation makes sense to me and the patch looks good.
>>
>> Reviewed-by: Javier Martinez Canillas <javierm at redhat.com>
>>
>