Re: [Linaro-mm-sig] [RFC v2 1/2] dma-buf: Introduce dma buffer sharing mechanism

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Dec 7, 2011 at 14:40, Arnd Bergmann <arnd@arndb.de> wrote:
> On Wednesday 07 December 2011, Semwal, Sumit wrote:
>> Thanks for the excellent discussion - it indeed is very good learning
>> for the relatively-inexperienced me :)
>>
>> So, for the purpose of dma-buf framework, could I summarize the
>> following and rework accordingly?:
>> 1. remove mmap() dma_buf_op [and mmap fop], and introduce cpu_start(),
>> cpu_finish() ops to bracket cpu accesses to the buffer. Also add
>> DMABUF_CPU_START / DMABUF_CPU_FINI IOCTLs?
>
> I think we'd be better off for now without the extra ioctls and
> just document that a shared buffer must not be exported to user
> space using mmap at all, to avoid those problems. Serialization
> between GPU and CPU is on a higher level than the dma_buf framework
> IMHO.

Agreed.

>> 2. remove sg_sync* ops for now (and we'll see if we need to add them
>> later if needed)
>
> Just removing the sg_sync_* operations is not enough. We have to make
> the decision whether we want to allow
> a) only coherent mappings of the buffer into kernel memory (requiring
> an extension to the dma_map_ops on ARM to not flush caches at map/unmap
> time)
> b) not allowing any in-kernel mappings (same requirement on ARM, also
> limits the usefulness of the dma_buf if we cannot access it from the
> kernel or from user space)
> c) only allowing streaming mappings, even if those are non-coherent
> (requiring strict serialization between CPU (in-kernel) and dma users of
> the buffer)

I think only allowing streaming access makes the most sense:
- I don't see much (if any need) for the kernel to access a dma_buf -
in all current usecases it just contains pixel data and no hw-specific
things (like sg tables, command buffers, ..). At most I see the need
for the kernel to access the buffer for dma bounce buffers, but that
is internal to the dma subsystem (and hence does not need to be
exposed).
- Userspace can still access the contents through the exporting
subsystem (e.g. use some gem mmap support). For efficiency reason gpu
drivers are already messing around with cache coherency in a platform
specific way (and hence violated the dma api a bit), so we could stuff
the mmap coherency in there, too. When we later on extend dma_buf
support so that other drivers than the gpu can export dma_bufs, we can
then extend the official dma api with already a few drivers with
use-patterns around.

But I still think that the kernel must not be required to enforce
correct access ordering for the reasons outlined in my other mail.
-Daniel
-- 
Daniel Vetter
daniel.vetter@ffwll.ch - +41 (0) 79 364 57 48 - http://blog.ffwll.ch

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