Messages in this thread | | | From | Eric Miao <> | Date | Mon, 9 Jan 2012 21:25:12 +0800 | Subject | Re: [PATCH v4 2/2] dma/imx-sdma: convert _raw_readl/_raw_writel to readl/writel |
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On Mon, Jan 9, 2012 at 7:51 PM, Russell King - ARM Linux <linux@arm.linux.org.uk> wrote: > On Sun, Jan 08, 2012 at 10:44:22PM +0800, Richard Zhao wrote: >> readl/writel is more genric. And if CONFIG_ARM_DMA_MEM_BUFFERABLE, >> they includes necessary memory barriers. > > In a DMA engine driver, you need to use the barrier accessors when: > > 1. You finally enable the DMA engine to perform a transfer. > The included barrier ensures that writes to the descriptors are visible > to the DMA engine. > > 2. You read from a status register before examining the descriptors. > This ensures that the descriptor accesses won't be ordered before the > status register read. > > Provided other accesses are within the same 1K region, the remainder of > them do not have to be the strictly ordered accessors, and you can use > the _relaxed variants (but only in ARM specific drivers.)
Russell,
Does this also mean when endian conversion is not necessary, the __raw_* version will be better here? Or generally the _relaxed variants are more recommended as endian conversion will be optimized away anyway with these AMBA accesses as both sides are little-endian?
> > So, if your DMA engine has a control register, and a descriptor pointer > register, you can write the descriptor pointer register with a > writel_relaxed(). When you write the control register to enable the > transfer, use writel() to ensure there's a barrier so the descriptors > are visible. > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel -- 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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