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@xxxxxxxxxx>
---
ChangeLog:
v2: - Minor updates after proofreading comments from Chris
v3: - Minor updates after more comments from Chris
v4: - Minor updates after comments from Randy
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..a9ba672 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 requests
diff --git a/Documentation/mmc/mmc-async-req.txt b/Documentation/mmc/mmc-async-req.txt
new file mode 100644
index 0000000..4877f29
--- /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
+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 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 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().
+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 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 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);
--
1.7.4.1
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