On Tue, 5 Jul 2011 21:43:28 +0200 Per Forlin wrote:
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
It would be better to omit the introductory email and put all of its comments in this one [PATCH] email.
Documentation/mmc/00-INDEX | 2 + Documentation/mmc/mmc-async-req.txt | 86 +++++++++++++++++++++++++++++++++++ 2 files changed, 88 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
requests
diff --git a/Documentation/mmc/mmc-async-req.txt b/Documentation/mmc/mmc-async-req.txt new file mode 100644 index 0000000..d7e7698 --- /dev/null +++ b/Documentation/mmc/mmc-async-req.txt @@ -0,0 +1,86 @@ +Rationale +=========
+How significant is the cache maintenance overhead? +It depends, fast eMMC and multiple cache levels with speculative cache
It depends: or It depends. Fast
+pre-fetch makes the cache overhead relatively significant. If the DMA +preparations for the next request are done in parallel to the current
with the current
+transfer the DMA preparation overhead would not affect the MMC performance.
transfer,
+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
mmc_wait_for_req(),
+dma_unmap_sg is processing. Using non-blocking mmc requests makes it
are processing.
+possible to prepare the caches for next job in parallel to an active
with an active
+mmc request.
+MMC block driver +================
+The issue_rw_rq() in the mmc block driver is made non-blocking.
preferably: MMC throughout the file (when not a function or data name, etc.)
+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)
are
+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.
with won't
+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()
mmc_start_req().
+It starts a new MMC command request for a host. The function isn't +truly non-blocking. If there is on 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 hooks pre_req() and post_req() that the host driver
hooks -- pre_req() and post_req() -- that
+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
DMA
+the dma_unmap_sg.
+Optimize for the first request +==============================
+The first request in a series of requests can't be prepared in parallel to
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)
Please use ';' at the end of each pseudo-call.
- /*
- 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)
--
--- ~Randy *** Remember to use Documentation/SubmitChecklist when testing your code ***