Re: [PATCH 2/5] dmaengine: dw: Activate FIFO-mode for memory peripherals only

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Thu, Jul 30, 2020 at 06:45:42PM +0300, Serge Semin wrote:
> CFGx.FIFO_MODE field controls a DMA-controller "FIFO readiness" criterion.
> In other words it determines when to start pushing data out of a DW
> DMAC channel FIFO to a destination peripheral or from a source
> peripheral to the DW DMAC channel FIFO. Currently FIFO-mode is set to one
> for all DW DMAC channels. It means they are tuned to flush data out of
> FIFO (to a memory peripheral or by accepting the burst transaction
> requests) when FIFO is at least half-full (except at the end of the block
> transfer, when FIFO-flush mode is activated) and are configured to get
> data to the FIFO when it's at least half-empty.
> 
> Such configuration is a good choice when there is no slave device involved
> in the DMA transfers. In that case the number of bursts per block is less
> than when CFGx.FIFO_MODE = 0 and, hence, the bus utilization will improve.
> But the latency of DMA transfers may increase when CFGx.FIFO_MODE = 1,
> since DW DMAC will wait for the channel FIFO contents to be either
> half-full or half-empty depending on having the destination or the source
> transfers. Such latencies might be dangerous in case if the DMA transfers
> are expected to be performed from/to a slave device. Since normally
> peripheral devices keep data in internal FIFOs, any latency at some
> critical moment may cause one being overflown and consequently losing
> data. This especially concerns a case when either a peripheral device is
> relatively fast or the DW DMAC engine is relatively slow with respect to
> the incoming data pace.
> 
> In order to solve problems, which might be caused by the latencies
> described above, let's enable the FIFO half-full/half-empty "FIFO
> readiness" criterion only for DMA transfers with no slave device involved.

> Thanks to the commit ???????????? ("dmaengine: dw: Initialize channel

See below.

> before each transfer") we can freely do that in the generic
> dw_dma_initialize_chan() method.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Thanks!

> Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
> 
> ---
> 
> Note the DMA-engine repository git.infradead.org/users/vkoul/slave-dma.git
> isn't accessible. So I couldn't find out the Andy' commit hash to use it in
> the log.

It's dmaengine.git on git.kernel.org.

> ---
>  drivers/dma/dw/dw.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/dma/dw/dw.c b/drivers/dma/dw/dw.c
> index 7a085b3c1854..d9810980920a 100644
> --- a/drivers/dma/dw/dw.c
> +++ b/drivers/dma/dw/dw.c
> @@ -14,7 +14,7 @@
>  static void dw_dma_initialize_chan(struct dw_dma_chan *dwc)
>  {
>  	struct dw_dma *dw = to_dw_dma(dwc->chan.device);
> -	u32 cfghi = DWC_CFGH_FIFO_MODE;
> +	u32 cfghi = is_slave_direction(dwc->direction) ? 0 : DWC_CFGH_FIFO_MODE;
>  	u32 cfglo = DWC_CFGL_CH_PRIOR(dwc->priority);
>  	bool hs_polarity = dwc->dws.hs_polarity;
>  
> -- 
> 2.27.0
> 

-- 
With Best Regards,
Andy Shevchenko