[PATCH v8 00/12] use nonblock mmc requests to minimize latency

[Poddar, Sourav]

On Tue, Jun 28, 2011 at 1:41 PM, Per Forlin <per.forlin at 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 non-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 non-blocking mmc request makes it
> possible to prepare the caches for next job in parallel to an active
> mmc request.
>
> This is done by making the issue_rw_rq() 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. 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 v7:
>  * rebase on mmc-next, on top of Russell's updated error handling.
>  * Clarify description of mmc_start_req()
>  * Resolve compile without CONFIG_DMA_ENIGNE issue for mmci
>  * Add mmc test to measure how performance is affected by sg length
>  * Add missing wait_for_busy in mmc_test non-blocking test. This call got lost
>   in v4 of this patchset when refactoring mmc_start_req.
>  * Add sub-prefix (core block queue) to relevant patches.
>
> Per Forlin (12):
>  mmc: core: add non-blocking mmc request function
>  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 non-blocking transfers
>  mmc: mmc_test: test to measure how sg_len affect performance
>  mmc: block: add member in mmc queue struct to hold request data
>  mmc: block: add a block request prepare function
>  mmc: block: move error code in issue_rw_rq to a separate function.
>  mmc: queue: add a second mmc queue request member
>  mmc: core: add random fault injection
>  mmc: block: add handling for two parallel block requests in
>    issue_rw_rq
>
>  drivers/mmc/card/block.c      |  505 ++++++++++++++++++++++++-----------------
>  drivers/mmc/card/mmc_test.c   |  491 ++++++++++++++++++++++++++++++++++++++--
>  drivers/mmc/card/queue.c      |  184 ++++++++++------
>  drivers/mmc/card/queue.h      |   33 ++-
>  drivers/mmc/core/core.c       |  167 +++++++++++++-
>  drivers/mmc/core/debugfs.c    |    5 +
>  drivers/mmc/host/mmci.c       |  147 +++++++++++-
>  drivers/mmc/host/mmci.h       |    8 +
>  drivers/mmc/host/omap_hsmmc.c |   87 +++++++-
>  include/linux/mmc/core.h      |    6 +-
>  include/linux/mmc/host.h      |   24 ++
>  lib/Kconfig.debug             |   11 +
>  12 files changed, 1345 insertions(+), 323 deletions(-)



Boot tested on Omap4430 Blaze board.

Tested-by: Sourav Poddar<sourav.poddar at ti.com>