Re: Forcing PIO mode instead of DMA via DT property

Dear Aggrwal Poonam-B10812,

quoted

-----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