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Date: Thu, 16 Jun 2011 19:09:25 +0530
Message-ID: <BANLkTi=v7nOaDmdUr2FscNtou-5vRhhmyw@mail.gmail.com>
Subject: Re: [PATCH v4 00/12] mmc: use nonblock mmc requests to minimize latency
From: "S, Venkatraman" <svenkatr@ti.com>
To: Per Forlin <per.forlin@linaro.org>
Cc: linux-mmc <linux-mmc@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linaro-dev <linaro-dev@lists.linaro.org>,
        David Vrabel <david.vrabel@csr.com>, Chris Ball <cjb@laptop.org>
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On Thu, May 26, 2011 at 3:27 AM, Per Forlin <per.forlin@linaro.org> wrote:
> How significant is the cache maintenance over head?
> It depends, the eMMC are much faster now
> compared to a few years ago and cache maintenance cost more due to
> multiple cache levels and speculative cache pre-fetch. In relation the
> cost for handling the caches have increased and is now a bottle neck
> dealing with fast eMMC together with DMA.
>
> The intention for introducing none blocking mmc requests is to minimize the
> time between a mmc request ends and another mmc request starts. In the
> current implementation the MMC controller is idle when dma_map_sg and
> dma_unmap_sg is processing. Introducing none blocking mmc request makes it
> possible to prepare the caches for next job parallel with an active
> mmc request.
>
> This is done by making the issue_rw_rq() none 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. Measurements on U5500
> and Panda on eMMC and SD shows significant performance gain for large
> reads when running DMA mode. In the PIO case the performance is unchanged.
>
> 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.
>
> Details on measurements from IOZone and mmc_test:
> https://wiki.linaro.org/WorkingGroups/Kernel/Specs/StoragePerfMMC-async-req
>
> Changes since v3:
> ?* Based on 2.6.39-rc7
> ?* Add error check for testlist in mmc_test.c
> ?* Resolve in mmc-queue-thread that caused the mmc-thread to miss a wakeup.
> ?* Move parallel request handling to core.c. This simplifies the interface
> ? from 4 public functions to 1. This also gives access for SDIO to use the
> ? same functionallity, even though the function is not tuned for the SDIO
> ? execution flow yet.
>
> Per Forlin (12):
> ?mmc: add none blocking mmc request function
> ?omap_hsmmc: use original sg_len for dma_unmap_sg
> ?omap_hsmmc: add support for pre_req and post_req
> ?mmci: implement pre_req() and post_req()
> ?mmc: mmc_test: add debugfs file to list all tests
> ?mmc: mmc_test: add test for none blocking transfers
> ?mmc: add member in mmc queue struct to hold request data
> ?mmc: add a block request prepare function
> ?mmc: move error code in mmc_block_issue_rw_rq to a separate function.
> ?mmc: add a second mmc queue request member
> ?mmc: test: add random fault injection in core.c
> ?mmc: add handling for two parallel block requests in issue_rw_rq
>
> ?drivers/mmc/card/block.c ? ? ?| ?452 +++++++++++++++++++++++++----------------
> ?drivers/mmc/card/mmc_test.c ? | ?361 ++++++++++++++++++++++++++++++++-
> ?drivers/mmc/card/queue.c ? ? ?| ?184 +++++++++++------
> ?drivers/mmc/card/queue.h ? ? ?| ? 32 +++-
> ?drivers/mmc/core/core.c ? ? ? | ?165 ++++++++++++++-
> ?drivers/mmc/core/debugfs.c ? ?| ? ?5 +
> ?drivers/mmc/host/mmci.c ? ? ? | ?146 ++++++++++++--
> ?drivers/mmc/host/mmci.h ? ? ? | ? ?8 +
> ?drivers/mmc/host/omap_hsmmc.c | ? 90 ++++++++-
> ?include/linux/mmc/core.h ? ? ?| ? ?6 +-
> ?include/linux/mmc/host.h ? ? ?| ? 19 ++
> ?lib/Kconfig.debug ? ? ? ? ? ? | ? 11 +
> ?12 files changed, 1187 insertions(+), 292 deletions(-)
>

Nitpick.. The mmc_test.c changes should be at the end of the series,
after the async feature is available.

Regards,
Venkat.
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