Re: [PATCH v2 2/3] drm/format-helper: Add drm_fb_xrgb8888_to_xrgb2101010_toio()

[Hector Martin <marcan@marcan.st>]

Hi, thanks for the review!

On 07/12/2021 18.40, Thomas Zimmermann wrote:
> Hi
> 
> Am 07.12.21 um 08:29 schrieb Hector Martin:
>> Add XRGB8888 emulation support for devices that can only do XRGB2101010.
>>
>> This is chiefly useful for simpledrm on Apple devices where the
>> bootloader-provided framebuffer is 10-bit.
>>
>> Signed-off-by: Hector Martin <marcan@marcan.st>
>> ---
>>    drivers/gpu/drm/drm_format_helper.c | 62 +++++++++++++++++++++++++++++
>>    include/drm/drm_format_helper.h     |  3 ++
>>    2 files changed, 65 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_format_helper.c b/drivers/gpu/drm/drm_format_helper.c
>> index dbe3e830096e..edd611d3ab6a 100644
>> --- a/drivers/gpu/drm/drm_format_helper.c
>> +++ b/drivers/gpu/drm/drm_format_helper.c
>> @@ -409,6 +409,59 @@ void drm_fb_xrgb8888_to_rgb888_toio(void __iomem *dst, unsigned int dst_pitch,
>>    }
>>    EXPORT_SYMBOL(drm_fb_xrgb8888_to_rgb888_toio);
>>    
>> +static void drm_fb_xrgb8888_to_xrgb2101010_line(u32 *dbuf, const u32 *sbuf,
>> +						unsigned int pixels)
>> +{
>> +	unsigned int x;
>> +
>> +	for (x = 0; x < pixels; x++) {
>> +		*dbuf++ = ((sbuf[x] & 0x000000FF) << 2) |
>> +			  ((sbuf[x] & 0x0000FF00) << 4) |
>> +			  ((sbuf[x] & 0x00FF0000) << 6);
> 
> This isn't quite right. The lowest two destination bits in each
> component will always be zero. You have to do the shifting as above and
> for each component the two highest source bits have to be OR'ed into the
> two lowest destination bits. For example the source bits in a component
> are numbered 7 to 0
> 
>    | 7 6 5 4 3 2 1 0 |
> 
> then the destination bits should be
> 
>    | 7 6 5 4 3 2 1 0 7 6 |
> 

I think both approaches have pros and cons. Leaving the two LSBs always 
at 0 yields a fully linear transfer curve with no discontinuities, but 
means the maximum brightness is slightly less than full. Setting them 
fully maps the brightness range, but creates 4 double wide steps in the 
transfer curve (also it's potentially slightly slower CPU-wise).

If you prefer the latter I'll do that for v2.

-- 
Hector Martin (marcan@marcan.st)
Public Key: https://mrcn.st/pub