Re: Forcing PIO mode instead of DMA via DT property

Dear Poonam,

In message [off-list ref] you wrote:

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).

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.

Best regards,

Wolfgang Denk

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