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author | Per Forlin <per.forlin@linaro.org> | 2011-07-10 23:21:59 +0400 |
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committer | Chris Ball <cjb@laptop.org> | 2011-07-21 18:34:52 +0400 |
commit | 7937e878f91ccc32c09177f44cfdc45183d78605 (patch) | |
tree | ea343c892288164d9ae8e963e25e832a20d70fcc /Documentation/mmc/mmc-async-req.txt | |
parent | 101ed47e01516adeffeb4769df77b9207e6ba48a (diff) | |
download | linux-7937e878f91ccc32c09177f44cfdc45183d78605.tar.xz |
mmc: documentation of mmc non-blocking request usage and design.
Documentation about the background and the design of mmc non-blocking.
Host driver guidelines to minimize request preparation overhead.
Signed-off-by: Per Forlin <per.forlin@linaro.org>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Chris Ball <cjb@laptop.org>
Diffstat (limited to 'Documentation/mmc/mmc-async-req.txt')
-rw-r--r-- | Documentation/mmc/mmc-async-req.txt | 87 |
1 files changed, 87 insertions, 0 deletions
diff --git a/Documentation/mmc/mmc-async-req.txt b/Documentation/mmc/mmc-async-req.txt new file mode 100644 index 000000000000..ae1907b10e4a --- /dev/null +++ b/Documentation/mmc/mmc-async-req.txt @@ -0,0 +1,87 @@ +Rationale +========= + +How significant is the cache maintenance overhead? +It depends. Fast eMMC and multiple cache levels with speculative cache +pre-fetch makes the cache overhead relatively significant. If the DMA +preparations for the next request are done in parallel with the current +transfer, the DMA preparation overhead would not affect the MMC performance. +The intention of non-blocking (asynchronous) MMC requests is to minimize the +time between when an MMC request ends and another MMC request begins. +Using mmc_wait_for_req(), the MMC controller is idle while dma_map_sg and +dma_unmap_sg are processing. Using non-blocking MMC requests makes it +possible to prepare the caches for next job in parallel with an active +MMC request. + +MMC block driver +================ + +The mmc_blk_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 are 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 with the transfer performance won't 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(). +It starts a new MMC command request for a host. The function isn't +truly non-blocking. If there is an ongoing async request it waits +for completion of that request and starts the new one and returns. It +doesn't wait for the new request to complete. If there is no ongoing +request it starts the new request and returns immediately. + +MMC host extensions +=================== + +There are two optional members in the mmc_host_ops -- pre_req() and +post_req() -- that the host driver may implement in order to move work +to before and after the actual mmc_host_ops.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 +with 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 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_2nd_chunk_for_dma(req); + /* + * 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); |