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>

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 requests

Documentation/mmc/mmc-async-req.txt

+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);