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

[Thomas Zimmermann <tzimmermann@suse.de>]

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Am 07.12.21 um 11:20 schrieb Thomas Zimmermann:
> Hi
> 
> Am 07.12.21 um 10:54 schrieb Hector Martin:
>> 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.
> 
> We don't give guarantees for color output unless color spaces are 
> involved. But the lack of LSB bits can be more visible than larger steps 
> in the curve. With the current formats here, it's probably a non-issue. 
> But there can be conversions, such as RGB444 to RGB88, where these 
> missing LSBs make a visible difference.
> 
> Therefore, please change the algorithm. It produces more consistent 
> results over a variety of format conversion. It's better to have the 
> same (default) algorithm for all of them.

FTR, I just tested this in a painting program. I can see a difference 
between ffffff and fcfcfc iff both are next to each other. f8f8f8 is 
obviously gray.

> 
> Best regards
> Thomas
> 
>>
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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