On 21 June 2011 21:18, Nicolas Pitre [off-list ref] wrote:

On Tue, 21 Jun 2011, Per Forlin wrote:

quoted

On 21 June 2011 07:41, Kishore Kadiyala [off-list ref] wrote:

quoted

<snip>

quoted

+
+static void omap_hsmmc_pre_req(struct mmc_host *mmc, struct mmc_request *mrq,
+ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?bool is_first_req)

I don't see the usage of "is_first_req" below.
Is it required?

It is not required. It is only an indication that this request is the
first in a series of request. The host driver may do various
optimisations based on this information. The first request in a series
of jobs can't be prepared in parallel to the previous job. The host
driver can do the following to minimise latency for the first job.
?* Preparing the cache while the MMC read/write cmd is being
processed. In this case the pre_req could do nothing and the job is
instead run in parallel to the read/write cmd being sent. If the
is_first_req is false pre_req will run in parallel to an active
transfer, in this case it is more efficient to prepare the request in
pre_req.
?* Run PIO mode instead of DMA

That is never going to be a good tradeoff. ?If the CPU is busy doing
PIO, it won't have a chance to prepare a subsequent request in parallel
to the first transfer.

If you have two CPUs and the MMC interrupts are scheduled on the CPU
1, CPU 0 can prepare the next one. I'm still in favor of the preparing
cache in parallel to cmd. I have run tests and for small req like 4k
there is a good performance gain. Another option, if the mmc
controller support it, would be to start with PIO and switch to DMA on
the fly when cache is ready. Bottom line, it is up to the host driver
to do something clever based on is_first_req.

/Per