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SubjectRe: [RESEND] spi: davinci: Allow device tree devices to use DMA
On 01/09/2017 01:48 PM, Mark Brown wrote:
> On Thu, Jan 05, 2017 at 09:26:17PM -0600, David Lechner wrote:
>> This allows SPI devices specified in a device tree to use DMA when the
>> master controller.
>> Since device tree is supposed to only describe the hardware, adding such
>> a configuration option to device tree would not be acceptable. So, this
>> is the best we can do for now to get SPI devices working with DMA.
>> Unfortunately, this excludes the possibility of using one SPI device with
>> DMA and one without on the same master.
> Why would you ever want to do that? What would ever make sense about
> not using DMA if it's available and the transfer is suitably large, or
> conversely why would one want to force DMA even if PIO would be more
> performant?

I don't particularly want to do that, but that is the way the
spi-davinci driver currently works. The choice between DMA or PIO is
specified in the platform data on a per-device basis.

What I get from your remarks is that this is wrong and it needs to be
fixed. If that is so, could someone please point out a driver that does
it the right way and I will try to fix it.

>> When I originally submitted this patch, there was some discussion as to whether
>> dspi->dma_rx should be changed to return an error rather than being null.
>> However, I prefer it the way it is and don't see a compelling reason to change
>> it.
> I don't know what the above comment means, sorry (and don't recall
> having seen any earlier versions of this).

FWIW, you can find the previous conversation at

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