RE: Forcing PIO mode instead of DMA via DT property

-----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 9:01 AM
To: Aggrwal Poonam-B10812
Cc: pavel-ynQEQJNshbs@public.gmane.org; devicetree-discuss-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org; Wolfgang Denk
Subject: Re: Forcing PIO mode instead of DMA via DT property

Dear Aggrwal Poonam-B10812,

quoted

quoted

-----Original Message-----
From: devicetree-discuss [mailto:devicetree-discuss-
bounces+poonam.aggrwal=freescale.com-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org] On Behalf Of
bounces+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).

quoted

quoted

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.

quoted

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 :-)

quoted

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.

Okay, if the choice to use PIO is for a specific controller(bus) because of hardware reasons, may be device tree could be a place.
But if it is simply that the user wants PIO mode for reasons like "less bytes to transfer" then device tree may not be the right place.

Regards
Poonam

quoted

Regards
Poonam

Best regards,
Marek Vasut
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