Hi,
I'm currently working on a new UAPI for Host1x/TegraDRM (see first draft
in thread "[RFC] Host1x/TegraDRM UAPI"[1]). One question that has come
up is regarding the buffer allocation mechanism. Traditionally, DRM
drivers provide custom GEM allocation IOCTLs. However, we now have DMA
Heaps, which would be sufficient for TegraDRM's needs, so we could skip
implementing any GEM IOCTLs in the TegraDRM UAPI, and rely on importing
DMA-BUFs. This would mean less code on TegraDRM's side.
However, one complication with using DMA Heaps is that it only provides
DMA-BUF FDs, so it is possible that a user application could run out of
free file descriptors if it is not adjusting its soft FD limit. This
would especially be a problem for existing applications that might have
worked with the traditional GEM model and didn't need to adjust their FD
limits, but would then fail in some situations with the increased FD
usage of DMA-BUF FDs.
My question is then: what is the role of DMA Heaps? If it is to be used
as a central allocator, should the FD issue be left to the application,
or addressed somehow? Should it be considered a potential alternative
for GEM allocations?
Thanks,
Mikko
[1] https://www.spinics.net/lists/dri-devel/msg262021.html
