Message-ID: <e991a548a16e8a40c68eedc860ce78c2013aace6.camel@mediatek.com>
Date:   Mon, 18 Sep 2023 10:46:58 +0000
From:   Yong Wu (吴勇) <Yong.Wu@...iatek.com>
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Subject: Re: [PATCH 3/9] dma-heap: Provide accessors so that in-kernel drivers
 can allocate dmabufs from specific heaps

On Tue, 2023-09-12 at 11:05 -0400, Nicolas Dufresne wrote:
>  	 
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
>  Le mardi 12 septembre 2023 à 08:47 +0000, Yong Wu (吴勇) a écrit :
> > On Mon, 2023-09-11 at 12:12 -0400, Nicolas Dufresne wrote:
> > >   
> > > External email : Please do not click links or open attachments
> until
> > > you have verified the sender or the content.
> > >  Hi,
> > > 
> > > Le lundi 11 septembre 2023 à 10:30 +0800, Yong Wu a écrit :
> > > > From: John Stultz <jstultz@...gle.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.
> 
> Ok, I see the benefit of having a common driver then. It would add to
> the
> complexity, but having a driver for the tee allocator and v4l2/heaps
> would be
> another option?

It's ok for v4l2. But our DRM also use this new heap and it will be
sent upstream in the next few days.

> 
> >   
> > 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.
> 
> From what I've read from Yunfei series, this is only partially true
> for V4L2.
> The vb2 queue MMAP feature have dmabuf exportation as optional, but
> its not a
> problem to always back it up with a dmabuf object. But for internal
> SHM buffers
> used for firmware communication, I've never seen any driver use a
> DMABuf.
> 
> Same applies for primary decode buffers when frame buffer compression
> or post-
> processing it used (or reconstruction buffer in encoders), these are
> not user
> visible and are usually not DMABuf.

If they aren't dmabuf, of course it is ok. I guess we haven't used
these. The SHM buffer is got by tee_shm_register_kernel_buf in this
case and we just use the existed dmabuf ops to complete SVP.

In our case, the vcodec input/output/working buffers and DRM input
buffer all use this new secure heap during secure video play.

> 
> > 
> > > 
> > > Nicolas
> > > 
> > > > 
> > > > Signed-off-by: John Stultz <jstultz@...gle.com>
> > > > Signed-off-by: T.J. Mercier <tjmercier@...gle.com>
> > > > Signed-off-by: Yong Wu <yong.wu@...iatek.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]
> 

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