Re: [PATCH] spi: imx: add module parameter to control DMA use

[Trent Piepho <tpiepho@xxxxxxxxxx>]

On Fri, 2019-03-08 at 12:16 +0000, Mark Brown wrote:
> On Fri, Mar 08, 2019 at 12:55:52AM +0000, Trent Piepho wrote:
> > On Thu, 2019-03-07 at 10:30 +0000, Mark Brown wrote:
> > > On Wed, Mar 06, 2019 at 06:57:12PM +0000, Trent Piepho wrote:
> > > > High speed receive operations may be less likely to have issues
> > > > with
> > > > FIFO overflow when using DMA than when using PIO.
> > > > The eCSPI appears to insert a 4 bit pause after each word in DMA
> > > > mode,
> > > > not done in PIO mode, which can make DMA transfers 50% slower than
> > > > PIO.
> > > Wow, that's...  innovative.  If that's the case can't the decision
> > > function for switching to DMA be tweaked to take into account slave
> > > mode operation instead?
> > Do you mean slave mode as in the imx being the spi slave and the
> > external device the master?  That's not being used here.
> 
> Sorry, not sure where I got that from.  Must've just switched from
> another mail talking about that or something.  The point still stands

I have another patch out for imx spi, to fix a buffer overflow related
to rx fifo flushing needed in spi slave mode.

> though, can't we handle this by adjusting the decision function?  Though
> there's an unfortunate system load/performance tradeoff...  If we can't
> do that then I think a runtime control with a device property in sysfs
> would be a good idea as well.

The module parameter is writable, so it is a runtime control in sysfs,
/sys/module/spi_imx/parameters/use_dma.  It's also settable on boot via
command line "spi_imx.use_dma=0".  Very handy if you have a kernel
driver that will use spi on init, which a custom sysfs device attr
wouldn't address, since it couldn't be set.  A device tree attribute
could be used, but this is Linux configuration, not hardware layout,
and shouldn't go into the device tree.

I also found another reason to not use DMA: qemu can't emulate imx SPI
when it uses DMA.  It works for PIO.  So being able to add
spi_imx.use_dma=0 to the kernel command line when booting the kernel in
qemu is very handy!