[PATCH v3 00/12] mmc: use nonblock mmc requests to minimize latency

[Philip Rakity]

On May 9, 2011, at 5:34 AM, Per Forlin wrote:

> On 9 May 2011 04:05, Philip Rakity <prakity at 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.

sdhci.c for ADMA and SDMA

spec is at
http://www.sdcard.org/developers/tech/sdcard/pls/simplified_specs/

version 3 discusses ADMA

> 
>> 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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>> 
>>