Forcing PIO mode instead of DMA via DT property

[Aggrwal Poonam-B10812]

> -----Original Message-----
> From: Wolfgang Denk [mailto:wd at denx.de]
> Sent: Monday, July 23, 2012 11:18 AM
> To: Aggrwal Poonam-B10812
> Cc: Marek Vasut; devicetree-discuss at lists.ozlabs.org; pavel at denx.de
> Subject: Re: Forcing PIO mode instead of DMA via DT property
> 
> Dear Poonam,
> 
> In message <ACB6D0C0104CFF42A45A5D82A0DD4F3D079B4DAE at 039-SN2MPN1-
> 013.039d.mgd.msft.net> you wrote:
> >
> > > 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.
> > 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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