On Mon, 18 Feb 2013, Arnd Bergmann wrote: > On Monday 18 February 2013, Guennadi Liakhovetski wrote: > > On Mon, 18 Feb 2013, Arnd Bergmann wrote: > > > > Sorry, I'm not sure how best to describe it, and I don't have sufficient > > information myself. In any case on a block-diagram of sh73a0 SDHI devices > > aren't directly connected to a common super-highway bus, instead they are > > on a bus-splitter. One more thing I forgot to mention - this error has > > been observed on SMP. > > Ah, I guess that explains it. If you have one CPU filling the buffer > and another CPU reading it, you need an smp_wmb/wmp_rmb pair. > > That case can easily happen with tmio interrupt handler in PIO > mode. Ok, I also managed to reproduce it on UP, and it definitely is related to MMC_CAP_POWER_OFF_CARD, but I'll have to do some more work on it. I asked Chris to hold back on this patch for now. Thanks Guennadi > > I checked clock code under drivers/sh/clk, I don't see any relevant > > __raw_* operations there. One I do see, however, GIC code does use > > *_relaxed() accessors, but under raw spinlocks... Do those provide > > sufficient barriers? > > A spinlock by definition contains an SMP barrier, which seems to be > what is missing here. I don't know if that is enough here, because > the spinlock only needs barriers to protect against other users > behind the same spinlock. > > In architecture independent code, any smp_wmb() on the writer side > must be paired with an smp_rmb on the reader. You probably have the > sufficient barriers on the side that initiates the I/O because > that uses a dmb() in the MMIO instructions, but that is only > by accident and not portable. > > Also, in the uniprocessor case, you have more barriers than you > need, which limits the performance. The best solution for > both performance and readability would be to use readw_relaxed() > and writew_relaxed() and manually add the barriers you actually > need, i.e. two smp_rmb()/smp_wmb() in the PIO case, and a > wmb() before you start a real DMA to the device and an rmb() > after a DMA from the device is complete. > > Arnd > --- Guennadi Liakhovetski, Ph.D. Freelance Open-Source Software Developer http://www.open-technology.de/ -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
