[RESEND PATCH v2] drm/mediatek: Add AFBC support to Mediatek DRM driver

[AngeloGioacchino Del Regno]

Il 12/10/22 16:12, Justin Green ha scritto:
>> These would be different from the macros that are available in bitfield.h, but
>> not *fundamentally* different, so these would look a little redundant...
>>
>> I think that you refer to that `pitch` variable that's coming from the DRM(/fb)
>> API... and bitfield macros are for register access... so I guess that one clean
>> way of avoiding the magic shifting (that is purely used to split the 32-bits
>> number in two 16-bits 'chunks') would be to perhaps use a union, so that you
>> will have something like u.pitch_lsb, u.pitch_msb (with lsb/msb being two u16).
> 
> Do you mean something like this?
> 
> union pitch_val {
>       struct split_pitch_val {
>            uint16_t lsb;
>            uint16_t msb;
>       } split;
>       uint32_t val;
> };
> 
> I think my concern with that approach would be it assumes the compiler
> packs structs tightly and it also assumes the endianness of the
> machine, whereas a bitshift is maybe more portable. Is this an issue
> worth considering since we know this driver will only run on specific
> MTK SoCs?

Yes I mean something like that (except, use u16, u32 short form)... and that
should be safe really. As for the endianness, a lot more would break already,
so I don't see that as a concern right now.