Re: How to enable DMA transfers for UART?

Li, Jun R

Hi Stefan and Erwan,

I’m rebasing my previous implementations to the latest master branch and you can find it here:

I’m still working on it since it is still lacking a couple of things and there are some bugs affecting RX. But the basic idea is to use UART IDLE to define the end of a RX DMA transaction.


If you think it is worth keeping going, let’s work together to make it happen.


Best Regards,







From: Erwan Gouriou <erwan.gouriou@...>
Date: Tuesday, March 26, 2019 at 08:10
To: Stefan Jaritz <stefan@...>
Cc: Paul Sokolovsky <paul.sokolovsky@...>, devel <devel@...>, John J Li <>
Subject: Re: [Zephyr-devel] How to enable DMA transfers for UART?


Hi Stefan,


On Tue, 26 Mar 2019 at 13:01, Stefan Jaritz <stefan@...> wrote:


I done a demo driver implementation:

Some problems I was facing:

- DMA API: the current one doesn't support ring buffer setups

- UART API: the current API is not mapping the IDLE event.

- UART API: no callbacks possible for event driven rx

With the current design of the UART and DMA API it seems to be
challenging to get a nice fusion.

So I was implementing a simple API for the demo, making block writes
possible and calling a callback when data is received. I didn't used the
UART and DMA API because it would became a dirty hack, while calling
functions form that API & overwriting/patching the ISRs. So I decided
for a lean straight forward implementation.

Thanks for sharing code and experience, this is useful!


In my case I am interested into SPI + DMA and USB ACM + DMA. From an
energy perspective it is much better to use DMA transfers for the
peripheral IOs (We can put the MCU to sleep).

Indeed, we should have a proper DMA driver that could be called from other drivers.


Maybe it is worth to put a thought on using DMA for peripherals by
default and only the special case is doing polling or RX/TX handling via
uart etc. ISRs.

While I agree DMA has numerous benefits, I don't think this should be the only configuration,

nor the default one. Main issue is initial complexity and configuration needs.

People should be able to use the async API first and then enable DMA once they feel comfortable

with the API.





On 20/03/2019 15:37, Paul Sokolovsky wrote:
> Hello Stefan,
> On Tue, 19 Mar 2019 15:18:02 +0000
> "Stefan Jaritz" <stefan@...> wrote:
>> Hey,
>> I am currently experimenting with ZephyrOS on different tasks. My
>> base is an own dev board which has a stm32f4xx MCU that connects
>> different ICs etc. via UART, I2C, SPI & GPIOs.
>> Currently the UART ISR is too slow to catch all chars at a rx burst.
>> Usually I am using UART + DMA transfers to relieve the MCU and save
>> energy.
> []
>> So I am asking for help or some example how to do an UART & DMA.
>> Think having an ringbuffer for rx and an intr firing after n Bytes
>> received via DMA. Also doing the tx via DMA would be perfect. Any
>> ideas to setup the Zephyr drivers to do so?
> Recently, a new "async" UART API was added to Zephyr, whose
> implementation could leverage DMA support (that was one of usecases for
> this API). There's an open ticket to implement it for STM32:
> You're welcome to cooperate with the STM32 maintainers on implementing
> it.
> []

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