[PATCH v9 05/11] drm/fourcc: Add DRM_FORMAT_X403

[Tomi Valkeinen]

Hi,

On 26/03/2026 16:43, Simon Ser wrote:
> On Wednesday, March 25th, 2026 at 15:02, Tomi Valkeinen <tomi.valkeinen at ideasonboard.com> wrote:
> 
>> +/*
>> + * 3 plane non-subsampled (444) YCbCr
>> + * 10 bpc, 30 bits per sample image data in a single contiguous buffer.
>> + * index 0: Y plane,  [31:0] x:Y2:Y1:Y0    [2:10:10:10] little endian
>> + * index 1: Cb plane, [31:0] x:Cb2:Cb1:Cb0 [2:10:10:10] little endian
>> + * index 2: Cr plane, [31:0] x:Cr2:Cr1:Cr0 [2:10:10:10] little endian
>> + */
>> +#define DRM_FORMAT_X403		fourcc_code('X', '4', '0', '3')
> 
> So, this one is different from the Q family, because Q has padding in

Any idea where the letters (P, Q, S) come from?

> LSB rather than MSB. Speaking of, maybe we should add "LSB aligned" to
> the doc comment to make that clear?

Yes, I can add that.

> Re-reading the sibling thread about DRM_FORMAT_XV20, sounds like the
> first digit matches my expectations for sub-sampling. How did you pick

I just used the name in Xilinx's BSP kernel.

> the last two digits? I think I would've expected "30" here rather than
> "03", since the last two planes are Cb Cr rather than Cr Cb.

Hmm, but X403 is Cb:Cr, and P030 is Cr:Cb, so doesn't 03 make sense 
here? Oh, but Q401 is Cr:Cb, and it's 01...

Now that I look at this... I think I have to go back and do more 
testing. From the Xilinx docs, it looks to me that the XV15/XV20 should 
have the same CrCb order than X403. But the comments in these patches 
say otherwise. I'm pretty sure my tests conformed to the comments here, 
but now I don't feel so sure anymore. It's been more than a year since I 
wrote the tests and properly tested these, so I have to spend a bit time 
to get everything up again.

> Has the first "X" letter been picked arbitrarily? It's already used to
> denote padding in other formats so I wonder if we should pick that
> instead of, say, "T".
I didn't invent the name, I just took the naming Xilinx used. I don't 
know the history behind it. I assume the "X" is for Xilinx, but I could 
be wrong here. What would "T" be for? "Tomi"? =)

  Tomi