Re: [PATCH v5 03/16] spi: dw: Locally wait for the DMA transactions completion

[Andy Shevchenko <andy.shevchenko@xxxxxxxxx>]

On Fri, May 29, 2020 at 7:02 AM Serge Semin
<Sergey.Semin@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> Even if DMA transactions are finished it doesn't mean that the SPI
> transfers are also completed. It's specifically concerns the Tx-only
> SPI transfers, since there might be data left in the SPI Tx FIFO after
> the DMA engine notifies that the Tx DMA procedure is done. In order to
> completely fix the problem first the driver has to wait for the DMA
> transaction completion, then for the corresponding SPI operations to be
> finished. In this commit we implement the former part of the solution.
>
> Note we can't just move the SPI operations wait procedure to the DMA
> completion callbacks, since these callbacks might be executed in the
> tasklet context (and they will be in case of the DW DMA). In case of
> slow SPI bus it can cause significant system performance drop.

I read commit message, I read the code. What's going on here since you
repeated xfer_completion (and its wait routine) from SPI core and I'm
wondering what happened to it? Why we are not calling
spi_finalize_current_transfer()?

...

>         dws->master->cur_msg->status = -EIO;
> -       spi_finalize_current_transfer(dws->master);
> +       complete(&dws->dma_completion);
>         return IRQ_HANDLED;

...

> +static int dw_spi_dma_wait(struct dw_spi *dws, struct spi_transfer *xfer)
> +{
> +       unsigned long long ms;
> +
> +       ms = xfer->len * MSEC_PER_SEC * BITS_PER_BYTE;
> +       do_div(ms, xfer->effective_speed_hz);
> +       ms += ms + 200;
> +
> +       if (ms > UINT_MAX)
> +               ms = UINT_MAX;
> +
> +       ms = wait_for_completion_timeout(&dws->dma_completion,
> +                                        msecs_to_jiffies(ms));
> +
> +       if (ms == 0) {
> +               dev_err(&dws->master->cur_msg->spi->dev,
> +                       "DMA transaction timed out\n");
> +               return -ETIMEDOUT;
> +       }
> +
> +       return 0;
> +}
> +
>  /*
>   * dws->dma_chan_busy is set before the dma transfer starts, callback for tx
>   * channel will clear a corresponding bit.
> @@ -155,7 +184,7 @@ static void dw_spi_dma_tx_done(void *arg)
>                 return;
>
>         dw_writel(dws, DW_SPI_DMACR, 0);
> -       spi_finalize_current_transfer(dws->master);
> +       complete(&dws->dma_completion);
>  }
>
>  static struct dma_async_tx_descriptor *dw_spi_dma_prepare_tx(struct dw_spi *dws,
> @@ -204,7 +233,7 @@ static void dw_spi_dma_rx_done(void *arg)
>                 return;
>
>         dw_writel(dws, DW_SPI_DMACR, 0);
> -       spi_finalize_current_transfer(dws->master);
> +       complete(&dws->dma_completion);
>  }


-- 
With Best Regards,
Andy Shevchenko