ast: resolutions that require single-buffering (due to VRAM limitations) are unavailable

[Thomas Zimmermann]

Hi

Am 25.10.22 um 09:12 schrieb Jeremy Rand:
> Hi dri-devel,
> 
> I have two machines with ASPEED GPU's (ast Linux driver).  One machine 
> is x86_64, running an ASRock Rack Tommy 90-SC02P1-00UBNZ GPU (AST2510 
> chipset) with KDE Plasma Wayland; the other is ppc64le, running an 
> integrated AST2500 GPU with KDE Plasma X11.  Both the AST2510 and 
> AST2500 have 16 MiB VRAM according to lspci.  Both ASPEED GPU's are 
> advertised as supporting up to 1920x1200 resolution, but KDE only 
> detects a maximum resolution of 1920x1080.
> 
> Some additional information about this bug can be found at 
> https://forums.raptorcs.com/index.php/topic,31.0.html .
> 
> I believe this is a Linux bug, because it is solely dependent on the 
> Linux version.  The following Linux versions are confirmed to have the bug:
> 
> Debian:
> 5.6.0-1 (ppc64el)
> Fedora:
> 5.6.0-1.fc33.x86_64
> 5.6.0-1.fc33.ppc64le
> 5.17.5-300.fc36.x86_64
> 5.18.6-200.fc36.ppc64le
> 6.1.0-0.rc0.20221007git4c86114194e6.5.fc38.ppc64le
> 
> Whereas the following Linux versions are confirmed to work fine (max 
> resolution detected by KDE is 1920x1200 as it should be, and that 
> resolution works fine when selected):
> 
> Debian:
> 5.5.0-2 (ppc64el)
> Fedora:
> 5.5.17-200.fc31.x86_64
> 5.5.17-200.fc31.ppc64le
> 
> I believe the bug was introduced by Linux commit 
> 9253f830c9166bfa6cc07d5ed59e174e9d5ec6ca, which adds a VRAM size check 
> that assumes double-buffering.  1920x1080 resolution at 4 bytes per 
> pixel with 2 buffers is 16.6 MB, while bumping that to 1920x1200 results 
> in 18.4 MB.  Since the VRAM size is 16 MiB == 16.8 MB, that explains the 
> issue.

Thanks for reporting. It's been a known issue for a while.

But in the most recent devel tree, we have replaced ast memory 
management, so that it can now use the full vram size for scanout 
buffers. See

 
https://cgit.freedesktop.org/drm/drm-tip/commit/drivers/gpu/drm/ast/ast_mode.c?id=f2fa5a99ca81ce1056539e83c705f3d6bec62e31

To test, get the latest drm-tip from

   git://anongit.freedesktop.org/drm/drm-tip

and try on your machine.

The updated driver should become available in Linux v6.3.

Best regards
Thomas

> 
> Steps to reproduce:
> 
> * Add a GPU that uses the ast driver, which supports 1920x1200 
> resolution and has 16 MiB VRAM.
> * Boot into KDE.
> * Go to KDE System Settings -> Display and Monitor -> Display 
> Configuration.
> * Look at list of detected resolutions.
> * KDE will only detect up to 1920x1080 resolution.
> 
> I'm attaching the output of "journalctl --no-hostname -k", although I'm 
> doubtful that anything useful is in the log.
> 
> I reported this bug to Fedora at 
> https://bugzilla.redhat.com/show_bug.cgi?id=2136950 , and was directed 
> to this mailing list.
> 
> Happy to provide more details on request.  (I'm not subscribed to this 
> list, so please CC any replies to me.)
> 
> Cheers,

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev
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