Re: [PATCH v5 05/16] spi: dw: Add SPI Rx-done wait method to DMA-based transfer

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

On Fri, May 29, 2020 at 12:46:48PM +0300, Andy Shevchenko wrote:
> On Fri, May 29, 2020 at 06:59:03AM +0300, Serge Semin wrote:
> > Having any data left in the Rx FIFO after the DMA engine claimed it has
> > finished all DMA transactions is an abnormal situation, since the DW SPI
> > controller driver expects to have all the data being fetched and placed
> > into the SPI Rx buffer at that moment. In case if this has happened we
> > assume that DMA engine still may be doing the data fetching, thus we give
> > it sometime to finish. If after a short period of time the data is still
> > left in the Rx FIFO, the driver will give up waiting and return an error
> > indicating that the SPI controller/DMA engine must have hung up or failed
> > at some point of doing their duties.
> 
> ...
> 
> > +static int dw_spi_dma_wait_rx_done(struct dw_spi *dws)
> > +{
> > +	int retry = WAIT_RETRIES;
> > +	struct spi_delay delay;
> > +	unsigned long ns, us;
> > +	u32 nents;
> > +
> > +	/*
> > +	 * It's unlikely that DMA engine is still doing the data fetching, but
> > +	 * if it's let's give it some reasonable time. The timeout calculation
> > +	 * is based on the synchronous APB/SSI reference clock rate, on a
> > +	 * number of data entries left in the Rx FIFO, times a number of clock
> > +	 * periods normally needed for a single APB read/write transaction
> > +	 * without PREADY signal utilized (which is true for the DW APB SSI
> > +	 * controller).
> > +	 */
> > +	nents = dw_readl(dws, DW_SPI_RXFLR);
> 

> > +	ns = NSEC_PER_SEC / dws->max_freq * 4 * nents;
> 
> I think we may slightly increase precision by writing this like
> 
> 	ns = 4 * NSEC_PER_SEC / dws->max_freq * nents;

Good point. Although both 4 and NSEC_PER_SEC are signed. The later is
1000000000L. Formally speaking on x32 systems (4 * 1000 000 000L) equals
to a negative value. Though overflow still won't happen so the result will
be correct. Anyway to be on a safe side it would be better to use an explicit
unsigned literal:

+       ns = 4U * NSEC_PER_SEC / dws->max_freq * nents;

-Sergey

> 
> 
> > +	if (ns <= NSEC_PER_USEC) {
> > +		delay.unit = SPI_DELAY_UNIT_NSECS;
> > +		delay.value = ns;
> > +	} else {
> > +		us = DIV_ROUND_UP(ns, NSEC_PER_USEC);
> > +		delay.unit = SPI_DELAY_UNIT_USECS;
> > +		delay.value = clamp_val(us, 0, USHRT_MAX);
> > +	}
> > +
> > +	while (dw_spi_dma_rx_busy(dws) && retry--)
> > +		spi_delay_exec(&delay, NULL);
> > +
> > +	if (retry < 0) {
> > +		dev_err(&dws->master->dev, "Rx hanged up\n");
> > +		return -EIO;
> > +	}
> > +
> > +	return 0;
> > +}
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>