[PATCH] mmc: documentation of mmc non-blocking request usage and design.

[Chris Ball]

Hi Per, minor proofreading,

On Tue, Jul 05 2011, Per Forlin wrote:
> Documentation about the background and the design of mmc non-blocking.
> Host driver guide lines to minimize request preparation over head.

guidelines, overhead

>
> Signed-off-by: Per Forlin <per.forlin at linaro.org>
> ---
>  Documentation/mmc/00-INDEX          |    2 +
>  Documentation/mmc/mmc-async-req.txt |   85 +++++++++++++++++++++++++++++++++++
>  2 files changed, 87 insertions(+), 0 deletions(-)
>  create mode 100644 Documentation/mmc/mmc-async-req.txt
>
> diff --git a/Documentation/mmc/00-INDEX b/Documentation/mmc/00-INDEX
> index 93dd7a7..11bc2cf 100644
> --- a/Documentation/mmc/00-INDEX
> +++ b/Documentation/mmc/00-INDEX
> @@ -4,3 +4,5 @@ mmc-dev-attrs.txt
>          - info on SD and MMC device attributes
>  mmc-dev-parts.txt
>          - info on SD and MMC device partitions
> +mmc-async-req.txt
> +        - info on mmc asynchronous request
> diff --git a/Documentation/mmc/mmc-async-req.txt b/Documentation/mmc/mmc-async-req.txt
> new file mode 100644
> index 0000000..d139a51
> --- /dev/null
> +++ b/Documentation/mmc/mmc-async-req.txt
> @@ -0,0 +1,85 @@
> +Rationale
> +=========
> +
> +How significant is the cache maintenance over head?

overhead

> +It depends, fast eMMC and multiple cache levels with speculative cache pre-fetch

This line is over 80 cols.  Please wrap at around 76 cols.

> +makes the cache overhead relatively significant. If the DMA preparations
> +for the next request is done in parallel to the current transfer

are done

> +the DMA preparation overhead would not affect the MMC performance.
> +The intention of non-blocking (asynchronous) mmc requests is to minimize the
> +time between a mmc request ends and another mmc request begins.

between when an

> +Using mmc_wait_for_req() the MMC controller is idle when dma_map_sg and

s/when/while/

> +dma_unmap_sg is processing. Using non-blocking mmc request makes it

requests

> +possible to prepare the caches for next job in parallel to an active
> +mmc request.
> +
> +MMC block driver
> +================
> +
> +The issue_rw_rq() in the mmc block driver is made 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. Roughly the expected
> +performance gain is 5% for large writes and 10% on large reads on a L2 cache
> +platform. In power save mode, when clocks run on a lower frequency, the DMA
> +preparation may cost even more. As long as these slower preparations are run
> +in parallel to the transfer performance wont be affected.
> +
> +Details on measurements from IOZone and mmc_test
> +================================================
> +
> +https://wiki.linaro.org/WorkingGroups/Kernel/Specs/StoragePerfMMC-async-req
> +
> +MMC core API extension
> +======================
> +
> +There is one new public function mmc_start_req()
> +Is starts a new MMC command request for a host. The function isn't

s/Is/It/

> +truely non-blocking. If there is on ongoing async request it waits

truly

> +for completion of that request and starts the new one and return. It

start

> +Doesn't wait for the new request to complete. If there is no ongoing

s/Doesn't/doesn't/

> +request it starts the new request and returns immediately.
> +
> +MMC host extensions
> +===================
> +
> +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.
> +
> +Optimize for the first request
> +==============================
> +
> +The first request in a series of requests can't be prepared in parallel to the
> +previous transfer, since there is no previous request.
> +The argument is_first_req in pre_req() indicates that there is no previous
> +request. The host driver may optimize for this scenario to minimize
> +the performance loss. A way to optimize for this is to split the current
> +request in two chunks, prepare the first chunk and start the request,
> +and finally prepare the second chunk and start the transfer.
> +
> +Pseudocode to handle is_first_req scenario with minimal prepare over head:

overhead

> +if (is_first_req && req->size > threshold)
> +   /* start MMC transfer for the complete transfer size */
> +   mmc_start_command(MMC_CMD_TRANSFER_FULL_SIZE)
> +
> +   /*
> +    * Begin to prepare DMA while cmd is being processed by MMC.
> +    * The first chunk of the request should take the same time
> +    * to prepare as the "MMC process command time".
> +    * If prepare time exceeds MMC cmd time
> +    * the transfer is delayed, guesstimate max 4k as first chunk size.
> +    */
> +    prepare_1st_chunk_for_dma(req)
> +    /* flush pending desc to the DMAC (dmaengine.h) */
> +    dma_issue_pending(req->dma_desc);
> +
> +    prepare_2st_chunk_for_dma(req)

2nd, not 2st

> +    /*
> +     * The second issue_pending should be called before MMC runs out
> +     * of the first chunk. If the MMC runs out of the first data chunk before
> +     * this call, the transfer is delayed.
> +     */
> +    dma_issue_pending(req->dma_desc);

Thanks,

- Chris.
-- 
Chris Ball   <cjb at laptop.org>   <http://printf.net/>
One Laptop Per Child