AM6442: AM64x UART DMA activation on Linux device tree

Hi Montassir,

The 0xc407/0x4407 pair is for MAIN UART5 on AM6442, if this is the UART port which you work with.

However, enabling DMA on UART ports is not supported in Linux, you might run into different UART data corruption problems.

Can you please explain the problem you have and why you want to enable DMA for UART?

Hello Bin,

The UART we use is UART1 (Ball E14/E15 on AM64).

Here is the description of the problem : Due to the nature of the data received on this UART, we cannot use HW flow control AND we must guarantee that data flow will be received at 1MBits/s or even more; whatever Linux is doing. To achieve that goal, we want to use DMA to move data from UART peripheral to memory; this is without A53 core CPU load.

On this AM64 Academy page, DMA seems supported by 8250 linux driver. As AM64 features 16C750 IP for UARTs, DMA support seems possible as 16C750 are 16C550 compatible and it is stated that DMA is supported:

Can you confirm that DMA is not possible on AM64 Linux; even for MAIN domain UARTs ?

Hi Bin, yes this was an example for SK board UART5.
According to your Linux SDK documentation, DMA is supported on UART : https://software-dl.ti.com/processor-sdk-linux/esd/AM64X/11_02_08_02/exports/docs/linux/Foundational_Components/Kernel/Kernel_Drivers/UART.html

Hi Montassir and Philippe,

Yes the UART on AM64x is capable with using DMA, however the problem is in the kernel UART driver (8250_omap.c) in which the DMA integration is not optimal for AM64x UART.

Philippe Bourgault said:

we cannot use HW flow control AND we must guarantee that data flow will be received at 1MBits/s or even more; whatever Linux is doing.

Have you tried to use irq affinity to assign the UART1 interrupt to CPU1 (by default it is handled by CPU0) to see if UART1 can receive data successfully without DMA?

I tried using DMA with UART and running iperf3 test with ppp : 
@ 1M baudrate : 
iperf3 TCP 400kbit/s for 1 hour in reception : 0 retries.

Seems to be working in a stable way. Do you think there is any risk in using DMA ? Or was it not fully validated by TI on am64x ?

Hi Montassir,

If you don't run into any issue, you can enable DMA on UART in your use case.

DMA on UART is not validated in Linux SDK.

montassir kounsi said:

+ dmas = <&main_pktdma 0xc407 15>, <&main_pktdma 0x4407 15>;

By the way, the number '15' above should be '0'. Please test with '0' to ensure you don't run into any issue too.

Thank you Bin,
What does 0 mean in this case ?

Bin Liu said:

Have you tried to use irq affinity to assign the UART1 interrupt to CPU1 (by default it is handled by CPU0) to see if UART1 can receive data successfully without DMA?

It's possible that we won't have 2 cpus in our final design, but only one.