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-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);