Re: [PATCH v8 00/12] use nonblock mmc requests to minimize latency

[Arnd Bergmann <arnd@arndb.de>]

On Tuesday 28 June 2011, Per Forlin wrote:
> How significant is the cache maintenance over head?
> It depends, the eMMC are much faster now
> compared to a few years ago and cache maintenance cost more due to
> multiple cache levels and speculative cache pre-fetch. In relation the
> cost for handling the caches have increased and is now a bottle neck
> dealing with fast eMMC together with DMA.
> 
> The intention for introducing non-blocking mmc requests is to minimize the
> time between a mmc request ends and another mmc request starts. In the
> current implementation the MMC controller is idle when dma_map_sg and
> dma_unmap_sg is processing. Introducing non-blocking mmc request makes it
> possible to prepare the caches for next job in parallel to an active
> mmc request.
> 
> This is done by making the issue_rw_rq() non-blocking.
> The increase in throughput is proportional to the time it takes to
> prepare (major part of preparations is dma_map_sg and dma_unmap_sg)
> a request and how fast the memory is. The faster the MMC/SD is
> the more significant the prepare request time becomes. Measurements on U5500
> and Panda on eMMC and SD shows significant performance gain for large
> reads when running DMA mode. In the PIO case the performance is unchanged.
> 
> There are two optional hooks pre_req() and post_req() that the host driver
> may implement in order to move work to before and after the actual mmc_request
> function is called. In the DMA case pre_req() may do dma_map_sg() and prepare
> the dma descriptor and post_req runs the dma_unmap_sg.

I think this looks good enough to merge into the linux-mmc tree, the code is
clean and the benefits are clear.

Acked-by: Arnd Bergmann <arnd@arndb.de>

One logical follow-up as both a cleanup and performance optimization would be
to get rid of the mmc_queue_thread completely. When mmc_blk_issue_rq() is
non-blocking always, you can call it directly from the mmc_request()
function, instead of waking up another thread to do it for you.

	Arnd