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SubjectRe: [PATCH v4 2/2] dma/imx-sdma: convert _raw_readl/_raw_writel to readl/writel
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.
>
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