[PATCH v6 02/11] omap_hsmmc: add support for pre_req and post_req

[Nicolas Pitre]

On Tue, 21 Jun 2011, Per Forlin wrote:

> On 21 June 2011 07:41, Kishore Kadiyala <kishorek.kadiyala at gmail.com> wrote:
> > <snip>
> >
> >> +
> >> +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.


Nicolas