On 3/25/22 13:32, Nicolas Dufresne wrote: > Le jeudi 24 mars 2022 à 21:20 +0300, Dmitry Osipenko a écrit : >> The root of the problem is that DRM UAPI is more flexible and allows to >> customize offsets for both S/MPLANEs, while V4L doesn't allow to do it >> at all. I'm exploring all the potential options, so far neither of the >> proposed variants is ideal. > > In GStreamer kmssink, the way DRM is used, is that if you have 2 planes in your > pixel format, but only received 1 DMABuf, we will pass this DMABuf twice (well > GEM handles, but twice), with appropriate offset. > > With this in mind, the idea for V4L2 could be to always resort to MPLANE for > this purpose. The tricky part for userland is that it needs to know the dual > pixel format and map that accordingly. That is a bit difficult and this is > something Helen was trying to address with the v4l2_buffer_ext (that and > allowing space to store DRM Modifiers in the future). FYI: here is Helen's last patch series. Since Helen is no longer active in the media subsystem, someone else who is sufficiently motivated would have to take over. https://patchwork.linuxtv.org/project/linux-media/cover/20210114180738.1758707-1-helen.koike@xxxxxxxxxxxxx/ I'm not enthusiastic about messing with data_offset: it was - in hindsight - a bad idea. Regards, Hans > > The second challenge is the overhead. In DRM, as we "prime" the DMABuf into > handles, this gives a kernel object to store any relevant information about the > buffer. So having it duplicate can be done at no cost. In V4L2, the driver would > need to handle that more often. Specially that despite the recommendation > (except for primary buffer decoder, were this is mandatory), we don't force a > strict DMABuf / Buffer IDX mapping. > > Nicolas
