RE: [RFC PATCH v2] dmabuf-sync: Introduce buffer synchronization framework

[Inki Dae <inki.dae@samsung.com>]

> -----Original Message-----
> From: dri-devel-bounces+inki.dae=samsung.com@lists.freedesktop.org
> [mailto:dri-devel-bounces+inki.dae=samsung.com@lists.freedesktop.org] On
> Behalf Of Russell King - ARM Linux
> Sent: Thursday, June 20, 2013 3:29 AM
> To: Inki Dae
> Cc: linux-fbdev; DRI mailing list; Kyungmin Park; myungjoo.ham; YoungJun
> Cho; linux-media@vger.kernel.org; linux-arm-kernel@lists.infradead.org
> Subject: Re: [RFC PATCH v2] dmabuf-sync: Introduce buffer synchronization
> framework
> 
> On Thu, Jun 20, 2013 at 12:10:04AM +0900, Inki Dae wrote:
> > On the other hand, the below shows how we could enhance the conventional
> > way with my approach (just example):
> >
> > CPU -> DMA,
> >         ioctl(qbuf command)              ioctl(streamon)
> >               |                                               |
> >               |                                               |
> >         qbuf  <- dma_buf_sync_get   start streaming <- syncpoint
> >
> > dma_buf_sync_get just registers a sync buffer(dmabuf) to sync object.
> And
> > the syncpoint is performed by calling dma_buf_sync_lock(), and then DMA
> > accesses the sync buffer.
> >
> > And DMA -> CPU,
> >         ioctl(dqbuf command)
> >               |
> >               |
> >         dqbuf <- nothing to do
> >
> > Actual syncpoint is when DMA operation is completed (in interrupt
> handler):
> > the syncpoint is performed by calling dma_buf_sync_unlock().
> > Hence,  my approach is to move the syncpoints into just before dma
> access
> > as long as possible.
> 
> What you've just described does *not* work on architectures such as
> ARMv7 which do speculative cache fetches from memory at any time that
> that memory is mapped with a cacheable status, and will lead to data
> corruption.

I didn't explain that enough. Sorry about that. 'nothing to do' means that a
dmabuf sync interface isn't called but existing functions are called. So
this may be explained again:
        ioctl(dqbuf command)
            |
            |
        dqbuf <- 1. dma_unmap_sg
                    2. dma_buf_sync_unlock (syncpoint)

The syncpoint I mentioned means lock mechanism; not doing cache operation.

In addition, please see the below more detail examples.

The conventional way (without dmabuf-sync) is:
Task A                             
----------------------------
 1. CPU accesses buf          
 2. Send the buf to Task B  
 3. Wait for the buf from Task B
 4. go to 1

Task B
---------------------------
1. Wait for the buf from Task A
2. qbuf the buf                 
    2.1 insert the buf to incoming queue
3. stream on
    3.1 dma_map_sg if ready, and move the buf to ready queue
    3.2 get the buf from ready queue, and dma start.
4. dqbuf
    4.1 dma_unmap_sg after dma operation completion
    4.2 move the buf to outgoing queue
5. back the buf to Task A
6. go to 1

In case that two tasks share buffers, and data flow goes from Task A to Task
B, we would need IPC operation to send and receive buffers properly between
those two tasks every time CPU or DMA access to buffers is started or
completed.


With dmabuf-sync:

Task A                             
----------------------------
 1. dma_buf_sync_lock <- synpoint (call by user side)
 2. CPU accesses buf          
 3. dma_buf_sync_unlock <- syncpoint (call by user side)
 4. Send the buf to Task B (just one time)
 5. go to 1


Task B
---------------------------
1. Wait for the buf from Task A (just one time)
2. qbuf the buf                 
    1.1 insert the buf to incoming queue
3. stream on
    3.1 dma_buf_sync_lock <- syncpoint (call by kernel side)
    3.2 dma_map_sg if ready, and move the buf to ready queue
    3.3 get the buf from ready queue, and dma start.
4. dqbuf
    4.1 dma_buf_sync_unlock <- syncpoint (call by kernel side)
    4.2 dma_unmap_sg after dma operation completion
    4.3 move the buf to outgoing queue
5. go to 1

On the other hand, in case of using dmabuf-sync, as you can see the above
example, we would need IPC operation just one time. That way, I think we
could not only reduce performance overhead but also make user application
simplified. Of course, this approach can be used for all DMA device drivers
such as DRM. I'm not a specialist in v4l2 world so there may be missing
point.

Thanks,
Inki Dae

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