[PATCH] mmci: restrict DMA usage to large, even multiblock transfers

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Tue, Feb 01, 2011 at 12:34:14PM +0100, Ulf Hansson wrote:
> Getting FIFO overrun/underruns is then as of today already the case when  
> running PIO for ARM primecell. To prevent this; you probably now that  
> already, hardware flow control is implemented in ST version of the ARM  
> primecell.

That really doesn't matter.  This driver is not specific to the ST
version.  It has to work on other versions as well.

I would like to see some further discussion on the unresolved issue of
the burst size setting - the discussion between myself and Linus died
without any apparant conclusion.  Linus tried to get some knowledgable
people from ST involved but no one ever followed up.

What I gathered from the discussion was that the additional DMA enable
bit found in ST variants should not be set as this changes the behaviour
of the DMA requests to be incompatible with the DMA controller
requirements.  With that bit clear, I see no problem with leaving the
burst threshold set at the half FIFO size.

That then makes avoiding DMA for requests below a specific size an
optimization issue and nothing more.