Role of DMA Heaps vs GEM in allocation

[Mikko Perttunen]

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