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

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi


Am 15.01.25 um 14:33 schrieb Tomi Valkeinen:
[...]
>> Yeah, there are constrains in the scanline and buffer alignments and 
>> orientation. And if we say that bpp==12 means NV12, it will be a 
>> problem for all other cases where bpp==12 makes sense.
>
> I feel I still don't quite understand. Can't we define and document 
> CREATE_DUMB like this:
>
> If (bpp < 8 || is_power_of_two(bpp))
>     bpp means bitsperpixel
>     pitch is args->width * args->bpp / 8, aligned up to 
> driver-specific-align
> else
>     bpp is a legacy parameter, and we deal with it case by case.
>     list the cases and what they mean
>
> And describe that when allocating subsampled buffers, the caller must 
> adjust the width and height accordingly. And that the bpp and width 
> can also refer to pixel groups.
>
> Or if the currently existing code prevents the above for 16 and 32 
> bpps, how about defining that any non-RGB or not-simple buffer has to 
> be allocated with bpp=8, and the userspace has to align the pitch 
> correctly according to the format and platform's hw restrictions?

What if a hardware requires certain per-format alignments? Or requires 
certain alignments for each plane? Or only supports tile modes? Or has 
strict limits on the maximum buffer size?

It is not possible to encode all this in a simple 32-bit value. So 
user-space code has to be aware of all this and tweak bpp-based 
allocation to make it work. Obviously you can use the current UAPI for 
your use case. It's just not optimal or future proof.

Best regards
Thomas

>
>
>  Tomi
>

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