-----Original Message----- From: Laura Abbott [mailto:lauraa@codeaurora.org] Sent: Friday, July 20, 2012 10:30 PM To: Marek Szyprowski Cc: linaro-mm-sig@lists.linaro.org; 'Russell King'; linux-arm-msm@vger.kernel.org; linux-arm- kernel@lists.infradead.org Subject: Re: [PATCH][RFC] arm: dma-mapping: Add support for allocating/mapping cached buffers
On 7/16/2012 10:58 PM, Marek Szyprowski wrote:
Hi Laura,
On Friday, July 13, 2012 8:02 PM Laura Abbott wrote:
There are currently no dma allocation APIs that support cached buffers. For some use cases, caching provides a signficiant performance boost that beats write-combining regions. Add apis to allocate and map a cached DMA region.
Signed-off-by: Laura Abbott lauraa@codeaurora.org
I agree that there is a need for cached contiguous memory blocks. I see that your patch is based on some older version of CMA/dma-mapping code. In v3.5-rc1 CMA has been merged to mainline kernel together with DMA-mapping redesign patches, so an attribute approach can be used instead of adding new functions to the API. My original idea was to utilize the dma_alloc_nonconsistent() call and DMA_ATTR_NONCONSISTENT for allocating/mapping cached contiguous buffers, but I didn't have enough time for completing this work.
The main missing piece is the API for managing cache synchronization on such buffers. There is a dma_cache_synch() functions but it is broken from the API point of view. To replace it with something better, some additional work is needed for all drivers which already use it. Also some work in needed for cleanup dma_alloc_nonconsistent() implementations for all the architectures using dma_map_ops approach. All this is on my TODO list, but I currently I'm really busy with other tasks related to CMA (mainly bugfixes for some special use-cases).
In what is the dma_cache_sync API broken? Just curious at this point.
There are two issues with it: 1. There is no clear buffer ownership definition like it is done for dma_sync_single_for_cpu/device() functions. 2. DMA address argument is missing, which is required for clean and robust implementation on some architectures.
I would like to completely remove dma_cache_sync() and replace it with dma_sync_single_for_cpu/device(), but this probably require a bit more discussion and fixing all current clients of dma_cache_sync().
Best regards