Re: [PATCH] mmc: documentation of mmc non-blocking request usage and design.

Hi Per, minor proofreading,

On Tue, Jul 05 2011, Per Forlin wrote:

Documentation about the background and the design of mmc non-blocking.
Host driver guide lines to minimize request preparation over head.

guidelines, overhead

overhead

+It depends, fast eMMC and multiple cache levels with speculative cache pre-fetch

This line is over 80 cols.  Please wrap at around 76 cols.

+makes the cache overhead relatively significant. If the DMA preparations
+for the next request is done in parallel to the current transfer

are done

+the DMA preparation overhead would not affect the MMC performance.
+The intention of non-blocking (asynchronous) mmc requests is to minimize the
+time between a mmc request ends and another mmc request begins.

between when an

+Using mmc_wait_for_req() the MMC controller is idle when dma_map_sg and

s/when/while/

+dma_unmap_sg is processing. Using non-blocking mmc request makes it

requests

+possible to prepare the caches for next job in parallel to 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 is 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 to 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()
+Is starts a new MMC command request for a host. The function isn't

s/Is/It/

+truely non-blocking. If there is on ongoing async request it waits

truly

+for completion of that request and starts the new one and return. It

start

+Doesn't wait for the new request to complete. If there is no ongoing

s/Doesn't/doesn't/

+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 to 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 over head:

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_2st_chunk_for_dma(req)

2nd, not 2st

+    /*
+     * 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);

Thanks,

- Chris.
-- 
Chris Ball   [off-list ref]   <http://printf.net/>
One Laptop Per Child

On 5 July 2011 17:24, Chris Ball [off-list ref] wrote:

Hi Per, minor proofreading,

Hi Chris,

Thanks for all your comments. I'll update and send out a v2.

Thanks,
Per