Re: [PATCH v2 25/25] drm/xlnx: Compute dumb-buffer sizes with drm_mode_size_dumb()

[Thomas Zimmermann <tzimmermann@xxxxxxx>]

Hi


Am 15.01.25 um 15:20 schrieb Tomi Valkeinen:
[...]
> My point is that we have the current UAPI, and we have userspace using 
> it, but we don't have clear rules what the ioctl does with specific 
> parameters, and we don't document how it has to be used.
> Perhaps the situation is bad, and all we can really say is that 
> CREATE_DUMB only works for use with simple RGB formats, and the 
> behavior for all other formats is platform specific. But I think even 
> that would be valuable in the UAPI docs.
To be honest, I would not want to specify behavior for anything but the 
linear RGB formats. If anything, I'd take Daniel's reply mail for 
documentation as-is. Anyone stretching the UAPI beyond RGB is on their own.
> Thinking about this, I wonder if this change is good for omapdrm or 
> xilinx (probably other platforms too that support non-simple non-RGB 
> formats via dumb buffers): without this patch, in both drivers, the 
> pitch calculations just take the bpp as bit-per-pixels, align it up, 
> and that's it.
> With this patch we end up using drm_driver_color_mode_format(), and 
> aligning buffers according to RGB formats figured out via heuristics. 
> It does happen to work, for the formats I tested, but it sounds like 
> something that might easily not work, as it's doing adjustments based 
> on wrong format.
> Should we have another version of drm_mode_size_dumb() which just 
> calculates using the bpp, without the drm_driver_color_mode_format() 
> path? Or does the drm_driver_color_mode_format() path provide some 
> value for the drivers that do not currently do anything similar?
With the RGB-only rule, using drm_driver_color_mode_format() makes 
sense. It aligns dumb buffers and video=, provides error checking, and 
overall harmonizes code. The fallback is only required because of the 
existing odd cases that already bend the UAPI's rules.
Best regards
Thomas

>  Tomi
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Thomas Zimmermann
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