[PATCH 1/3] spi/qspi: Add memory mapped read support.

[Mark Brown]

On Wed, Oct 09, 2013 at 07:27:11PM -0700, Trent Piepho wrote:

> I've found that the SPI layer adds rather a lot of overhead to SPI
> transactions.  It appears to come mostly from using another thread to
> run the queue.  A fast SPI message of a few dozen bytes ends up having
> more overhead from the SPI layer than the time it takes the driver to
> do the actual transfer.

Yeah, though of course at the minute the implementation of that thread
is pretty much up to the individual drivers which isn't triumph - and
the quality of implementation does vary rather a lot.  I'm currently
working on trying to factor more of this out, hopefully then it'll be
easier to push out improvements.  It may be nice to be able to kick off
the first DMA transfer from within the caller for example.

> So memory mapped mode via some kind of SPI hack seems like a bad
> design.  All the SPI layer overhead and you don't get DMA.  Memory
> mapped SPI could be a win, but I think you'd need to do it at the MTD
> layer with a mapping driver that could read the mmapped SPI flash
> directly.

Yes, exactly and even then I'm not convinced that it's going to be much
advantage for anything except small transfers without DMA.
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