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

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

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.