[PATCH] drm/fbdev-dma: Add shadow buffering for deferred I/O

[Javier Martinez Canillas]

Simona Vetter <simona.vetter at ffwll.ch> writes:

> On Wed, Dec 11, 2024 at 10:06:28AM +0100, Thomas Zimmermann wrote:
>> DMA areas are not necessarily backed by struct page, so we cannot
>> rely on it for deferred I/O. Allocate a shadow buffer for drivers
>> that require deferred I/O and use it as framebuffer memory.
>> 
>> Fixes driver errors about being "Unable to handle kernel NULL pointer
>> dereference at virtual address" or "Unable to handle kernel paging
>> request at virtual address".
>> 
>> The patch splits drm_fbdev_dma_driver_fbdev_probe() in an initial
>> allocation, which creates the DMA-backed buffer object, and a tail
>> that sets up the fbdev data structures. There is a tail function for
>> direct memory mappings and a tail function for deferred I/O with
>> the shadow buffer.
>> 
>> It is no longer possible to use deferred I/O without shadow buffer.
>> It can be re-added if there exists a reliably test for usable struct
>> page in the allocated DMA-backed buffer object.
>> 
>> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
>> Reported-by: Nuno Gonçalves <nunojpg at gmail.com>
>> CLoses: https://lore.kernel.org/dri-devel/CAEXMXLR55DziAMbv_+2hmLeH-jP96pmit6nhs6siB22cpQFr9w@mail.gmail.com/
>> Tested-by: Nuno Gonçalves <nunojpg at gmail.com>
>> Fixes: 5ab91447aa13 ("drm/tiny/ili9225: Use fbdev-dma")
>> Cc: Thomas Zimmermann <tzimmermann at suse.de>
>> Cc: <stable at vger.kernel.org> # v6.11+
>
> fbdev code scares me, but I at least tried to check a few things and looks
> all good.
>
> Reviewed-by: Simona Vetter <simona.vetter at ffwll.ch>
>

Same here, is always scary to review fbdev code but the patch looks good to me.

Reviewed-by: Javier Martinez Canillas <javierm at redhat.com>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat