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    SubjectRe: [PATCH v3 00/12] mmc: use nonblock mmc requests to minimize latency
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    On 9 May 2011 04:05, Philip Rakity <prakity@marvell.com> wrote:
    >
    > Hi Per,
    >
    > We noticed on some of our systems if we ADMA or SDMA and a bounce buffer it is significantly faster then SDMA.
    >
    I have not done work with ADMA or SDMA. Where should I look to read
    more about it?
    Are these the right places. DMA iop-dma.c and imx-sdma.c, MMC: sdhci.c.

    > I believe ADMA will do large transfers.  Another data point.
    >
    > Philip
    Thanks,
    Per

    >
    > On May 7, 2011, at 12:14 PM, Per Forlin 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 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
    >>
    >> Under consideration:
    >> * Make pre_req and post_req private to core.c.
    >> * Generalize implementation and make it available for SDIO.
    >>
    >> Changes since v2:
    >> * Fix compile warnings in core.c and block.c
    >> * Simplify max transfer size in mmc_test
    >> * set TASK_RUNNING in queue.c before issue_req()
    >>
    >> Per Forlin (12):
    >>  mmc: add none blocking mmc request function
    >>  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: add handling for two parallel block requests in issue_rw_rq
    >>  mmc: test: add random fault injection in core.c
    >>  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()
    >>
    >> drivers/mmc/card/block.c      |  493 +++++++++++++++++++++++++++--------------
    >> drivers/mmc/card/mmc_test.c   |  340 +++++++++++++++++++++++++++-
    >> drivers/mmc/card/queue.c      |  180 ++++++++++------
    >> drivers/mmc/card/queue.h      |   31 ++-
    >> drivers/mmc/core/core.c       |  132 ++++++++++-
    >> 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      |    9 +-
    >> include/linux/mmc/host.h      |   13 +-
    >> lib/Kconfig.debug             |   11 +
    >> 12 files changed, 1174 insertions(+), 284 deletions(-)
    >>
    >> --
    >> 1.7.4.1
    >>
    >> --
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    >
    >
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