[v2,5/6] drm/fb-helper: Schedule deferred-I/O worker after writing to framebuffer

[Marek Szyprowski]

On 17.11.2022 17:07, Thomas Zimmermann wrote:
> Am 17.11.22 um 13:57 schrieb Marek Szyprowski:
>> On 15.11.2022 12:58, Thomas Zimmermann wrote:
>>> Schedule the deferred-I/O worker instead of the damage worker after
>>> writing to the fbdev framebuffer. The deferred-I/O worker then performs
>>> the dirty-fb update. The fbdev emulation will initialize deferred I/O
>>> for all drivers that require damage updates. It is therefore a valid
>>> assumption that the deferred-I/O worker is present.
>>>
>>> It would be possible to perform the damage handling directly from 
>>> within
>>> the write operation. But doing this could increase the overhead of the
>>> write or interfere with a concurrently scheduled deferred-I/O worker.
>>> Instead, scheduling the deferred-I/O worker with its regular delay of
>>> 50 ms removes load off the write operation and allows the deferred-I/O
>>> worker to handle multiple write operations that arrived during the 
>>> delay
>>> time window.
>>>
>>> v2:
>>>     * keep drm_fb_helper_damage() (Daniel)
>>>     * use fb_deferred_io_schedule_flush() (Daniel)
>>>     * clarify comments (Daniel)
>>>
>>> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
>>> Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
>>
>> This patch, merged into today's linux-next as commit 7f5cc4a3e5e4
>> ("drm/fb-helper: Schedule deferred-I/O worker after writing to
>> framebuffer"), triggers a following warning on Raspberry Pi 3 & 4 as
>> well as all Amlogic Meson G12A/B based boards:
>>
>> ------------[ cut here ]------------
>> WARNING: CPU: 0 PID: 220 at drivers/video/fbdev/core/fb_defio.c:340
>
> Thank you so much for reporting. That line should never be executed 
> with vc4 et al.
>
> If you have the time, could you please try the attached patch and 
> report the results. Thanks a lot.

This fixes the issue observed on my Raspberry Pi 3/4 and Amlogic Meson 
based boards. Feel free to add:

Tested-by: Marek Szyprowski <m.szyprowski at samsung.com>

Best regards

-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland