at91: serial: Can't use DMA on 4.x kernels in RS-485 mode on at91sam9x5

[Alexandre Belloni]

Hi,

On 11/04/2017 at 18:32:20 +0000, Bryan Evenson wrote:
> I narrowed my issues down to this commit: https://github.com/linux4sam/linux-at91/commit/0058f0871efe7b01c6f2b3046c68196ab73e96da 
> 
> 	tty/serial: atmel: fix RS485 half duplex with DMA
> 
> 	When using DMA, half duplex doesn't work properly because rx is not stopped
> 	before starting tx. Ensure we call atmel_stop_rx() in the DMA case.
> 
> 	Signed-off-by: Alexandre Belloni <alexandre.belloni at free-electrons.com>
> 	Acked-by: Nicolas Ferre <nicolas.ferre at atmel.com>
> 	Cc: stable <stable at vger.kernel.org>
> 	Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> 
> If I revert this commit, I can use both TX and RX DMA on all three RS-485 ports (USART0, USART1 and USART3) with the current head of linux-4.4-at91 and linux-4.1-at91 on Atmel's fork.  I have not yet tried the mainline releases, but I see that atmel_start_tx() (the function modified by this commit) is identical in mainline so I'm assuming I'll have the same results.  In my case I am using my RS-485 ports in half-duplex mode, so the call to atmel_stop_rx() inside of atmel_start_tx() is the cause of my problems.
> 
> Anyone know why this would be a problem?  The commit makes sense to me and I don't see a good reason why it would break things.  I'm willing to test things out to see why this is causing a problem.
> 

Do you have b389f173aaa1204d6dc1f299082a162eb0491545 in your tree ?


-- 
Alexandre Belloni, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com