[PATCH] arm: Improve MMC performance on Versatile Express

[Nicolas Pitre]

On Fri, 21 Jan 2011, Russell King - ARM Linux wrote:

> On Fri, Jan 21, 2011 at 02:59:35PM -0500, Nicolas Pitre wrote:
> > On Fri, 21 Jan 2011, Pawel Moll wrote:
> > 
> > > VE suffers from serious problem with MMC transfers - low performance,
> > > errors when other IO peripherals (especially USB) are used at the
> > > same time etc.
> > > 
> > > It all boils down to the MMC controller short FIFO and - in result -
> > > timing constrains. The most problematic case - USB driver hogging
> > > CPU and MMC FIFO under/overruns in the result - can be mitigated on
> > > SMP system by distributing interrupts handling for these peripherals
> > > between cores.
> > 
> > Wouldn't the ultimate solution be to simply use FIQs to service the MMC FIFO?
> 
> Could you suggest how to route an arbitary interrupt to the FIQ using
> the GIC?

No I can't, hence my question.

> On systems which implement security extensions, the non-secure world can
> only use IRQs and not FIQs.  The secure world can use FIQs - in which
> case interrupts marked as secure interrupts can go to FIQ.

In that case, in theory, the secure world could provide some kind of 
software based DMA engine facility to the non-secure world for this FIFO 
servicing purpose.

> On systems without security extensions, the GIC only supports IRQ, and
> you need a second GIC implemented for FIQs.  I'm not aware of any system
> doing that.

The only solution in that case is to give top priority to the FIFO IRQ 
and never disable IRQs when in interrupt context, except for that FIFO 
servicing handler which should keep IRQs masked out throughout.  In any 
case this would certainly be only a hack for badly misdesigned hardware.


Nicolas