Re: Forcing PIO mode instead of DMA via DT property

[Marek Vasut <marex-ynQEQJNshbs@public.gmane.org>]

Dear Aggrwal Poonam-B10812,

> > -----Original Message-----
> > From: devicetree-discuss [mailto:devicetree-discuss-
> > bounces+poonam.aggrwal=freescale.com-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org] On Behalf Of Marek
> > Vasut
> > Sent: Monday, July 23, 2012 2:58 AM
> > To: devicetree-discuss-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org
> > Cc: pavel-ynQEQJNshbs@public.gmane.org; Wolfgang Denk
> > Subject: Forcing PIO mode instead of DMA via DT property
> > 
> > Hi guys,
> > 
> > I'm trying to implement a driver that can do both DMA and PIO, and it
> > would be nice if the user was able to select the mode (on a per-bus
> > basis) using the DT.
> > The PIO mode can reduce the overhead in some cases and therefore be
> > better choice than the DMA (for example when most transfers move only
> > very few data, or when board-specific hardware properties kick in).
> > 
> > I was thinking about using some "manf,use-pio" DT property, but I haven't
> > found any such example yet, so I wonder if this is a good idea.
> 
> Not sure, but as far as I understand device tree is to publish the hardware
> capabilities, not the user choice/configuration.

This might actually be one of the things I've been missing about DT, so thanks 
for stating that clearly :-)

> One option would be to
> put a compile time flag in the driver, which prefers PIO than DMA. I
> believe by default DMA is the first preference.

Certainly, but please consider you might want to use PIO only on selected 
instances of the driver (aka. selected busses, I wasn't probably clear enough, 
this is a bus driver), not degrade performance on all of them. Possibly due to a 
specific hardware wiring of the particular system.

> Regards
> Poonam

Best regards,
Marek Vasut