DMA support for UART on RPI

Martin Sperl kernel at
Sun Mar 6 02:55:21 PST 2016

> On 06.03.2016, at 11:43, Stefan Wahren <stefan.wahren at> wrote:
> Hi,
> inspired by Martin's latest patch series ("Patch v3: dmaengine: bcm2835: bugfix
> + enhancement of driver") i noticed that the BCM2835 has DREQ signals for UART.
> Does it mean at least one UART on the RPI has DMA support?
> Which one is it (PL011 / Mini UART)?

Unfortunately the documentation on the uart on page 175 says:

	The following functionality is not supported :

	*  Infrared Data Association (IrDA)

	*  Serial InfraRed (SIR) protocol Encoder/Decoder (ENDEC)

	*  Direct Memory Access (DMA).

So it was planned but eventually left out.

But it may also be true that the Documentation is wrong - we have seen
that before as well.

On the other hand they define the DMACR register in the documentation
on page 193, but there it says:
This is the disabled DMA Control Register, writing to it has not effect
and reading returns 0.


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