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.