Forcing PIO mode instead of DMA via DT property

[Wolfgang Denk]

Dear Arnd,

In message <201207241319.45101.arnd at arndb.de> 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.
> > 
> 
> What kind of device is this? We are currently working on the dmaengine
> binding, so an easy way to do this would be (one that binding is complete)
> to either provide or not provide the channel description depending on
> what you want to do with the device. This is clearly a hack but might
> fit your use case without adding any ugly code to the kernel.
> 
> Another option would be to make it a runtime configuration option,
> e.g. through sysfs, but that again depends a lot on what device you
> are talking about.

At least in my example of the x86 system a sysfs interface would not
help, as the kernel would crash during bootup before I can run user
space code.

Best regards,

Wolfgang Denk

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