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

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 10.07.23 um 11:52 schrieb Javier Martinez Canillas:
> Thomas Zimmermann <tzimmermann@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@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@suse.de>
>> Tested-by: Moritz Duge <MoritzDuge@kolahilft.de>
>> Tested-by: Torsten Krah <krah.tm@gmail.com>
>> Tested-by: Paul Schyska <pschyska@gmail.com>
>> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
>> Cc: David Airlie <airlied@gmail.com>
>> Cc: Noralf Trønnes <noralf@tronnes.org>
>> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
>> Cc: Maxime Ripard <mripard@kernel.org>
>> Cc: Javier Martinez Canillas <javierm@redhat.com>
>> Cc: Russell King <linux@armlinux.org.uk>
>> Cc: Inki Dae <inki.dae@samsung.com>
>> Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
>> Cc: Kyungmin Park <kyungmin.park@samsung.com>
>> Cc: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
>> Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
>> Cc: Rob Clark <robdclark@gmail.com>
>> Cc: Abhinav Kumar <quic_abhinavk@quicinc.com>
>> Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
>> Cc: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>> Cc: Alex Deucher <alexander.deucher@amd.com>
>> Cc: "Christian König" <christian.koenig@amd.com>
>> Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
>> Cc: Thierry Reding <thierry.reding@gmail.com>
>> Cc: Mikko Perttunen <mperttunen@nvidia.com>
>> Cc: dri-devel@lists.freedesktop.org
>> Cc: linux-kernel@vger.kernel.org
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-samsung-soc@vger.kernel.org
>> Cc: linux-arm-msm@vger.kernel.org
>> Cc: freedreno@lists.freedesktop.org
>> Cc: amd-gfx@lists.freedesktop.org
>> Cc: linux-tegra@vger.kernel.org
>> Cc: dri-devel@lists.freedesktop.org
>> Cc: <stable@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.

Best regards
Thomas

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

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

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