[U-Boot] Access IO register with writel/readl?

[Scott Wood <scottwood@freescale.com>]

On 10/18/2012 07:09:19 AM, Simon Glass wrote:
> Hi,
> 
> On Wed, Oct 17, 2012 at 6:47 PM, Dennis Lan (dlan)
> <dennis.yxun@gmail.com> wrote:
> > HI ALL:
> >    Is it just conventional to access IO register with wriltel,  
> readl?
> > or is there any specific reason here.
> > what's the difference with direct access? writel has few arch  
> releated
> > operation to guarantee hardware access?
> 
> Yes it is a convention. For CPUs which need to introduce
> compiler/memory barriers or other operations, this is supposed to be
> done automatically in the access functions rather than in the code.
> 
> Volatile tells the compiler that the value can change, and not to
> optimise accesses to it, but this is just the compiler and does not
> necessarily do everything required for I/O access on the platform.
> Linux has a README on this topic (search for
> Documentation/volatile-considered-harmful.txt) but I'm not sure that
> the locking / multi-threading discussion has much relevance to U-Boot.
> 
> (Oddly, I did notice at one point that on ARM some of the access
> functions don't actually have any barriers. It might have been
> clrsetbits.)

It looks like ARM is missing barriers on out_le32 etc.  At one point  
ARM was missing barriers on all accessors, and commit  
3c0659b535b075be124c3d2a0714e55e65c46737 only bothered with the  
writel/readb/etc. accessors.

> People tend to use writel() rather than __raw_writel(). In U-Boot with
> a flat address mapping I'm not sure there is much difference in
> practice, but it is nice to keep code consistent.

writel() is a little endian access with memory barriers if needed by  
the architecture.  __raw_writel() is native-endian and does not provide  
barriers.  Unfortunately there doesn't seem to be an accessor that is  
native-endian with barriers or specified-endian without barriers.

At least, that's what happens on powerpc, and I don't see anything  
obviously contradicting that from scanning some other arches.

-Scott