Re: [PATCH 3/9] dma-heap: Provide accessors so that in-kernel drivers can allocate dmabufs from specific heaps

["Yong Wu (吴勇)" <Yong.Wu@mediatek.com>]

On Mon, 2023-09-11 at 12:12 -0400, Nicolas Dufresne wrote:
>  	 
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>  Hi,
> 
> Le lundi 11 septembre 2023 à 10:30 +0800, Yong Wu a écrit :
> > From: John Stultz <jstultz@google.com>
> > 
> > This allows drivers who don't want to create their own
> > DMA-BUF exporter to be able to allocate DMA-BUFs directly
> > from existing DMA-BUF Heaps.
> > 
> > There is some concern that the premise of DMA-BUF heaps is
> > that userland knows better about what type of heap memory
> > is needed for a pipeline, so it would likely be best for
> > drivers to import and fill DMA-BUFs allocated by userland
> > instead of allocating one themselves, but this is still
> > up for debate.
> 
> 
> Would be nice for the reviewers to provide the information about the
> user of
> this new in-kernel API. I noticed it because I was CCed, but
> strangely it didn't
> make it to the mailing list yet and its not clear in the cover what
> this is used
> with. 
> 
> I can explain in my words though, my read is that this is used to
> allocate both
> user visible and driver internal memory segments in MTK VCODEC
> driver.
> 
> I'm somewhat concerned that DMABuf objects are used to abstract
> secure memory
> allocation from tee. For framebuffers that are going to be exported
> and shared
> its probably fair use, but it seems that internal shared memory and
> codec
> specific reference buffers also endup with a dmabuf fd (often called
> a secure fd
> in the v4l2 patchset) for data that is not being shared, and requires
> a 1:1
> mapping to a tee handle anyway. Is that the design we'd like to
> follow ? 

Yes. basically this is right.

> Can't
> we directly allocate from the tee, adding needed helper to make this
> as simple
> as allocating from a HEAP ?

If this happens, the memory will always be inside TEE. Here we create a
new _CMA heap, it will cma_alloc/free dynamically. Reserve it before
SVP start, and release to kernel after SVP done.
  
Secondly. the v4l2/drm has the mature driver control flow, like
drm_gem_prime_import_dev that always use dma_buf ops. So we can use the
current flow as much as possible without having to re-plan a flow in
the TEE.

> 
> Nicolas
> 
> > 
> > Signed-off-by: John Stultz <jstultz@google.com>
> > Signed-off-by: T.J. Mercier <tjmercier@google.com>
> > Signed-off-by: Yong Wu <yong.wu@mediatek.com>
> > [Yong: Fix the checkpatch alignment warning]
> > ---
> >  drivers/dma-buf/dma-heap.c | 60 ++++++++++++++++++++++++++++----
> ------
> >  include/linux/dma-heap.h   | 25 ++++++++++++++++
> >  2 files changed, 69 insertions(+), 16 deletions(-)
> > 
[snip]