RE: Forcing PIO mode instead of DMA via DT property

-----Original Message-----
From: Wolfgang Denk [mailto:wd-ynQEQJNshbs@public.gmane.org]
Sent: Monday, July 23, 2012 11:18 AM
To: Aggrwal Poonam-B10812
Cc: Marek Vasut; devicetree-discuss-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org; pavel-ynQEQJNshbs@public.gmane.org
Subject: Re: Forcing PIO mode instead of DMA via DT property

Dear Poonam,

In message <ACB6D0C0104CFF42A45A5D82A0DD4F3D079B4DAE@039-SN2MPN1-
013.039d.mgd.msft.net> you wrote:

quoted

quoted

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.
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.

We just had such a situation, where PIO had to be used on a specific bus,
because due to a hardware problem (board design error) DMA would crash
the system.  Yet the same driver/kernel would work fine with DMA on other
buses / boards.  OK, this was on x86, so DT would not have helped, but I
think a bus-specific property of being able to do DMA (or not) might
indeed make sense for the DT.

In that case probably we can use QUIRKS like broken-dma. I see this in the sdhc driver in Linux.
On P2020 chip(rev 1.0) of FSL DMA was broken so we used this to force PIO.

Regards
Poonam

Best regards,

Wolfgang Denk

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