Re: [PATCH 1/3] add dma_coherent_write_sync to DMA API

Josh Cartwright
2011-09-01T02:32:45Z

On Wed, Aug 31, 2011 at 05:30:12PM -0400, Mark Salter wrote:
> On ARMv6/7 DMA-coherent memory is bufferable which means that CPU writes to
> coherent memory may still be held in a write buffer for a significant amount
> of time. This is largely mitigated by having the MMIO write functions force
> a write buffer flush before doing the actual write to the MMIO register. This
> forces out previous CPU writes to coherent memory for drivers which write to
> a register to inform the device that something was written to memory. However,
> this does not mitigate the problem for devices which poll the DMA memory for
> changes written by the CPU. One such case was found by ming.lei@canonical.com
> in the USB EHCI driver. The EHCI host controller relies at least partly on
> polling DMA coherent memory for information from the driver.
>
> This patch adds a dma_coherent_write_sync() function to the DMA API which
> drivers can use to explicitly force out data which may otherwise be held up
> in a write buffer. It is a no-op unless and architecture provides its own
> version or the function and sets ARCH_HAS_DMA_COHERENT_WRITE_SYNC.
>
> Signed-off-by: Mark Salter <msalter@redhat.com>
> ---
> Documentation/DMA-API-HOWTO.txt | 15 +++++++++++++++
> Documentation/DMA-API.txt | 12 ++++++++++++
> include/linux/dma-mapping.h | 6 ++++++
> 3 files changed, 33 insertions(+), 0 deletions(-)
>
> diff --git a/Documentation/DMA-API-HOWTO.txt b/Documentation/DMA-API-HOWTO.txt
> index a0b6250..8c22b8b 100644
> --- a/Documentation/DMA-API-HOWTO.txt
> +++ b/Documentation/DMA-API-HOWTO.txt
> @@ -400,6 +400,21 @@ Make sure you've called dma_pool_free for all memory allocated
> from a pool before you destroy the pool. This function may not
> be called in interrupt context.
>
> +Some architectures which supporting DMA coherent memory may still have write
> +buffering between the CPU and DMA memory. This buffering may delay CPU writes
> +from reaching coherent memory in a timely manner. These delays in turn can
> +lead lead to dramatic performance issues in certain cases. An architecture

'lead lead' -> 'lead'

--
joshc