Messages in this thread | | | Date | Wed, 11 Mar 2009 15:46:05 +0000 | From | Matt Fleming <> | Subject | Re: [PATCH 5/7] mmc_spi: allow higher timeouts for SPI mode |
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On Wed, Mar 11, 2009 at 03:55:00PM +0100, Wolfgang Mües wrote: > > My patch 6 in mmc_spi_skip() is doing a busy-wait for a short while ( less > than 1 jiffie), and starts to call schedule() inside the loop if the card is > slower. >
OK, but if my machine runs at 100 HZ then a jiffie is 10ms. Previously (without your patch) we waited for 300ms in the write case and 100ms in the read case. So, it's not unreasonable to think that a response is going to take more than 10ms. With your patch we're almost always going to schedule() with no indication of exactly when we're going to come back.
> My goal was to avoid the long-lasting busy waiting. I have measured times up > to 900ms! With my patch, the longest busy waiting will be 1 jiffie. >
I agree that busy-waiting for 900ms would be a bit mad. Is there a reason that you didn't implement this with msleep() as was noted in the comment above the timeout?
/* REVISIT investigate msleep() to avoid busy-wait I/O * in at least some cases. */
> And yes, if the SD card is sending its response after 5 jiffies, it is > recognized only after the scheduler schedules this process, which will incure > a delay to the data transfer. The amount of delay is determined by the number > of running processes and the number of HZ. >
Have you benchmarked this case? Do you know approximately how long it is before we return from the schedule() under various workloads? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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