[PATCH 10/10] drivers/spi: add driver for the Multichannel SPI controller found in TI SoCs

[Jan Lübbe]

Hi,

On Tue, 2012-09-04 at 10:58 +0200, Sascha Hauer wrote:
> > +     for (i = 0; i < len; i++) {
> > +             /* wait till TX register is empty (TXS == 1) */
> > +             while
> (!(readl(&regs->channel[spi->chip_select].chstat) &
> > +                      OMAP3_MCSPI_CHSTAT_TXS)) {
> > +                     if (--timeout <= 0) {
> > +                             printf("SPI TXS timed out, status=0x%
> 08x\n",
> > +
> readl(&regs->channel[spi->chip_select].chstat));
> > +                             return -1;
> > +                     }
> > +             }
> 
> Please use a well defined timeout rather than 'poll a million times' 

I tried using is_timeout like this:
                timer_start = get_time_ns();
                while (!(readl(&regs->channel[spi->chip_select].chstat) &
                         OMAP3_MCSPI_CHSTAT_TXS)) {
                        if (is_timeout(timer_start, SPI_WAIT_TIMEOUT)) {
                                printf("SPI TXS timed out, status=0x%08x\n",
                                       readl(&regs->channel[spi->chip_select].chstat));
                                return -1;
                        }
                }

With SPI_WAIT_TIMEOUT = MSECOND this results in ~23% more time needed
for reading/writing:

With a simple counter:
barebox at KSP-0500:/ time cp /dev/m25p0.norkernel /dev/null
time: 12639ms
barebox at KSP-0500:/ time cp /dev/m25p0.norkernel /dev/null
time: 12639ms
barebox at KSP-0500:/ time cp /dev/m25p0.norkernel /dev/null
time: 12659ms

With is_timeout():
barebox at KSP-0500:/ time cp /dev/m25p0.norkernel /dev/null
time: 15561ms
barebox at KSP-0500:/ time cp /dev/m25p0.norkernel /dev/null
time: 15564ms
barebox at KSP-0500:/ time cp /dev/m25p0.norkernel /dev/null
time: 15568ms

Is this really acceptable? Is there a way to avoid the overhead of
calling several functions for each loop?

Regards,
Jan
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