Re: [PATCH v3 01/16] spi: dw: Add Tx/Rx finish wait methods to the MID DMA

[Serge Semin <Sergey.Semin@baikalelectronics.ru>]

Hello Feng,

On Thu, May 21, 2020 at 11:09:24AM +0800, Feng Tang wrote:
> Hi Serge,
> 
> On Thu, May 21, 2020 at 04:21:51AM +0300, Serge Semin wrote:

[nip]

> >  /*
> >   * dws->dma_chan_busy is set before the dma transfer starts, callback for rx
> >   * channel will clear a corresponding bit.
> > @@ -200,6 +267,8 @@ static void dw_spi_dma_rx_done(void *arg)
> >  {
> >  	struct dw_spi *dws = arg;
> >  
> > +	dw_spi_dma_wait_rx_done(dws);
> 
> I can understand the problem about TX, but I don't see how RX
> will get hurt, can you elaborate more? thanks
> 
> - Feng

Your question is correct. You are right with your hypothesis. Ideally upon the
dw_spi_dma_rx_done() execution Rx FIFO must be already empty. That's why the
commit log signifies the error being mostly related with Tx FIFO. But
practically there are many reasons why Rx FIFO might be left with data:
DMA engine failures, incorrect DMA configuration (if DW SPI or DW DMA driver
messed something up), controller hanging up, and so on. It's better to catch
an error at this stage while propagating it up to the SPI device drivers.
Especially seeing the wait-check implementation doesn't gives us much of the
execution overhead in normal conditions. So by calling dw_spi_dma_wait_rx_done()
we make sure that all the data has been fetched and we may freely get the
buffers back to the client driver.

-Sergey