SAMA5D3x: I2C, USART1 and DMA.

[sylvain.rochet@finsecur.com (Sylvain Rochet)]

Hi Peter,

On Tue, Nov 24, 2015 at 05:44:24PM +0100, Sylvain Rochet wrote:
> Hi Peter,
> 
> On Tue, Nov 24, 2015 at 05:14:15PM +0100, Peter Rosin wrote:
> > Hi!
> > 
> > I have a board similar to the atmel sama5d31ek with some devices
> > on the i2c0 bus and an async serial line on usart1 that communicates
> > with a baudrate of 125000. The usart is mostly receiving.
> > 
> > In a divine moment, our designers failed to add handshaking signals
> > for the usart, and now we have trouble with the occational lost interrupt
> > and hence lost data (at least that is my current understanding of
> > what is going on).
> > 
> > The lost data is clearly tied to i2c traffic, and specifically to i2c writes.
> > i2c reads seems to go by unnoticed by usart1. Of course, other stuff
> > may also cause trouble, but if I test by temporarily switching off the
> > i2c writes a BIG part of the problem is gone. But the i2c writes also
> > have a reason to be there of course, so that is not a long term
> > solution...
> > 
> > What immediately springs to mind is to reduce the number of interrupts
> > needed on the usart by enabling DMA. DMA is apparently disabled by
> > arch/arm/boot/dts/sama5d3xmb.dtsi with this:
> > 
> >             usart1: serial at f0020000 {
> >                 dmas = <0>, <0>;    /*  Do not use DMA for usart1 */
> > 
> > However, cutting out the "dmas" line does not improve things. So, how
> > do I enable DMA on usart1?
> 
> You are probably running out of available DMA channels on the DMAC0. 
> There is 8 channels available per DMAC and this is quite a scarce 
> resource.
> 
> See "DMA Channels Definition" tables from the SAMA5D3 datasheet. I hope 
> you balanced well the peripherals you are using in your design on the 
> two DMAC to prevent running out of DMA channels.

I forgot to add that you need to explicitely enable DMA on usart as 
explained in the Documentation/devicetree/bindings/serial/atmel-usart.txt
documentation, this way: 

usart0: serial at f001c000 {
        ...
        atmel,use-dma-rx;
        atmel,use-dma-tx;
        ...
}

Sylvain