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

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

Hi,

On 16/01/2025 10:09, Thomas Zimmermann wrote:
> 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.

I have to disagree here.

On the platforms I have been using (omap, tidss, xilinx, rcar) the dumb 
buffers are the only buffers you can get from the DRM driver. The dumb 
buffers have been used to allocate linear and multiplanar YUV buffers 
for a very long time on those platforms.

I tried to look around, but I did not find any mentions that CREATE_DUMB 
should only be used for RGB buffers. Is anyone outside the core 
developers even aware of it?

If we don't use dumb buffers there, where do we get the buffers? Maybe 
from a v4l2 device or from a gpu device, but often you don't have those. 
DMA_HEAP is there, of course.

So we have the option to get DMA_HEAP buffers, specifying just the size 
of the buffer. Here we only specify the size, so the userspace has to 
understand the requirements for the format and the platform.

Or we can use CREATE_DUMB, specifying the width, height and 
bitsperpixel, and if we don't have any heuristics about figuring out the 
pixel format (as it has been), the end result is exactly the same as 
with DMA_HEAP (i.e. we essentially define the size of the buffer).

So, on these platforms (omap, tidss, xilinx, rcar), the CREATE_DUMB has 
always been just "give me X amount of memory that can be used for 
scanout". With this series, the meaning of the ioctl changes, and it's 
now "give me an memory buffer buffer that works with an RGB format with 
this width, height, bpp".

In practice I believe that doesn't cause regressions, as aligning 
buffers according to RGB pixel format rules happens to be fine for YUV 
formats too, but I'm not sure (and it already almost caused a regression 
with bpp=64). And I'm having trouble seeing the upside.

Aligning video= and dumb buffers almost sounds like going backwards. 
video= parameter is bad, so let's also make dumb buffers bad?

Harmonizing code is fine, but I think that can be done with a function 
that only does the fallback-case.

So... I can only speak for the platforms I'm using and maintaining, but 
I'd rather keep the old behavior for CREATE_DUMB that we've had for ages.

  Tomi