On Fri, 25 Aug 2023 16:04:18 +0200 Jocelyn Falempe <jfalempe@xxxxxxxxxx> wrote: > After discussions on IRC, the consensus is that the DRM drivers should > avoid software color conversion, and only advertise the formats supported > by hardware. > Update the doc accordingly so that the rule and exceptions are clear for > everyone. > > Acked-by: Simon Ser <contact@xxxxxxxxxxx> > Signed-off-by: Jocelyn Falempe <jfalempe@xxxxxxxxxx> > --- > drivers/gpu/drm/drm_plane.c | 24 ++++++++++++++++++++++++ > 1 file changed, 24 insertions(+) > > diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c > index 24e7998d1731..d05642033202 100644 > --- a/drivers/gpu/drm/drm_plane.c > +++ b/drivers/gpu/drm/drm_plane.c > @@ -140,6 +140,30 @@ > * DRM_FORMAT_MOD_LINEAR. Before linux kernel release v5.1 there have been > * various bugs in this area with inconsistencies between the capability > * flag and per-plane properties. > + * > + * All drivers should support XRGB8888, even if the hardware cannot support > + * it. This has become the de-facto standard and a lot of user-space assume > + * it will be present. If XRGB8888 is not natively supported, then it > + * shouldn't be the default for preferred depth or fbdev emulation. > + * > + * DRM drivers should not do software color conversion, and > + * only advertise the formats they support in hardware. This is for > + * performance reason, and to avoid multiple conversions in userspace and > + * kernel space. KMS page flips are generally expected to be very cheap > + * operations. > + * > + * But there are two exceptions only for dumb buffers: > + * * To support XRGB8888 if it's not supported by the hardware. > + * * Any driver is free to modify its internal representation of the format, > + * as long as it doesn't alter the visible content in any way, and doesn't > + * modify the user-provided buffer. An example would be to drop the > + * padding component from a format to save some memory bandwidth. > + * On most hardware, VRAM read access are slow, so when doing the software > + * conversion, the dumb buffer should be allocated in system RAM in order to > + * have decent performance. > + * Extra care should be taken when doing software conversion with > + * DRM_CAP_DUMB_PREFER_SHADOW, there are more detailed explanations here: > + * https://lore.kernel.org/dri-devel/20230818162415.2185f8e3@eldfell/ > */ Acked-by: Pekka Paalanen <pekka.paalanen@xxxxxxxxxxxxx> Thanks, pq
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