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

[Richard Zhao <richard.zhao@linaro.org>]

On Mon, Jan 09, 2012 at 11:51:43AM +0000, Russell King - ARM Linux 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.)
> 
> 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.
Thanks very much for teaching. I understand now what relaxed means here.
I'll change all register access to _relaxed version except in function
sdma_enable_channel.
Freescale power pc also has sdma, I guess we can use _relaxed till they
tend to use the same driver ( I don't know when).

Thanks
Richard