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

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

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.)

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.