Re: [PATCH v5] mmc: documentation of mmc non-blocking request usage and design.

[J Freyensee <james_p_freyensee@linux.intel.com>]

On 07/05/2011 11:30 PM, Per Forlin wrote:
> Documentation about the background and the design of mmc non-blocking.
> Host driver guidelines to minimize request preparation overhead.

Resending this out on the linux-mmc list since that is what I am 
subscribed to (and I had html format on so original got blocked).

I'd like to make a couple suggestions on the documentation when 
documenting actual function names.  In general, really state the name of 
the function.  See below for issues.

> Signed-off-by: Per Forlin<per.forlin@linaro.org>
> Acked-by: Randy Dunlap<rdunlap@xenotime.net>
> ---
> ChangeLog:
>   v2: - Minor updates after proofreading comments from Chris
>   v3: - Minor updates after more comments from Chris
>   v4: - Minor updates after comments from Randy
>   v5: - Fixed one more comment and Acked-by from Randy
>
>   Documentation/mmc/00-INDEX          |    2 +
>   Documentation/mmc/mmc-async-req.txt |   86 +++++++++++++++++++++++++++++++++++
>   2 files changed, 88 insertions(+), 0 deletions(-)
>   create mode 100644 Documentation/mmc/mmc-async-req.txt
>
> diff --git a/Documentation/mmc/00-INDEX b/Documentation/mmc/00-INDEX
> index 93dd7a7..a9ba672 100644
> --- a/Documentation/mmc/00-INDEX
> +++ b/Documentation/mmc/00-INDEX
> @@ -4,3 +4,5 @@ mmc-dev-attrs.txt
>           - info on SD and MMC device attributes
>   mmc-dev-parts.txt
>           - info on SD and MMC device partitions
> +mmc-async-req.txt
> +        - info on mmc asynchronous requests
> diff --git a/Documentation/mmc/mmc-async-req.txt b/Documentation/mmc/mmc-async-req.txt
> new file mode 100644
> index 0000000..b7a52ea
> --- /dev/null
> +++ b/Documentation/mmc/mmc-async-req.txt
> @@ -0,0 +1,86 @@
> +Rationale
> +=========
> +
> +How significant is the cache maintenance overhead?
> +It depends. Fast eMMC and multiple cache levels with speculative cache
> +pre-fetch makes the cache overhead relatively significant. If the DMA
> +preparations for the next request are done in parallel with the current
> +transfer, the DMA preparation overhead would not affect the MMC performance.
> +The intention of non-blocking (asynchronous) MMC requests is to minimize the
> +time between when an MMC request ends and another MMC request begins.
> +Using mmc_wait_for_req(), the MMC controller is idle while dma_map_sg and
> +dma_unmap_sg

if dma_unmap_sg/dma_map_sg are complete functions, please
> are processing. Using non-blocking MMC requests makes it
> +possible to prepare the caches for next job in parallel with an active
> +MMC request.
> +
> +MMC block driver
> +================
> +
> +The issue_rw_rq() in the MMC block driver is made non-blocking.
Could this be made *_issue_rw_rq() please?  When I see 'issue_rw_rq()', 
I assume it is referring to an entire function with that name.  But I am 
really thinking this is for functions ending with '_issue_rw_rq()', 
right?  Like in mmc_blk_issue_rw_rq()?

Actually, if mmc_blk_issue_rw_rq() is the only function, please just use 
this.
> +The increase in throughput is proportional to the time it takes to
> +prepare (major part of preparations are 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 with the transfer performance won't 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 it really meant mmc_start_req*uest*()?  That is what I see in core.c.

Also, is this the actual async API being introduced in this work that is 
to be used by client drivers?  I don't see it being exported with 
EXPORT_SYMBOL()/EXPORT_SYMBOL_GPL() like mmc_request_done() is in the 
linux-next tree (and I just recently pulled it because I had to fix my 
own driver bug :-/).
> +It starts a new MMC command request for a host. The function isn't
> +truly non-blocking. If there is on ongoing async request it waits
> +for completion of that request and starts the new one and returns. It
> +doesn't wait for the new request to complete. If there is no ongoing
> +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
Same here...pre_req()/post_req()...are these functions meant to have 
'pre_req()' in the name?  Please use *_pre_req().  Otherwise, just state 
the exact function name.
> +driver may implement in order to move work to before and after the actual
> +mmc_request function is called.
If there is only a couple of mmc request functions being referred to 
here, please just type it out.
> 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 with
> +the previous transfer, since there is no previous request.
> +The argument is_first_req in pre_req() indicates that there is no previous
Minor thing...if 'is_first_req' a function or macro, please add the '()' 
to it.

And please use *_pre_req()/type-out-exact-pre_req() function please.
> +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 overhead:
Please add a blank line here after the 'Pseduocode' statement.  I'm only 
suggesting it because there are blank lines in the pseudo-code itself to 
help improve readability.
> +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_2nd_chunk_for_dma(req);
> +    /*
> +     * 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);