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

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

[Dennis Lan]

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?


  for example:

struct base_gpt {
        unsigned int control;
        unsigned int prescaler;
};

#define GPT1_BASE_ADDR GPTCR_SWR
volatile struct base_gpt *cur_gpt = (struct base_gpt *)GPT1_BASE_ADDR;

A) acess with writel
        __raw_writel(GPTCR_SWR, &cur_gpt->control);

B) direct access
       cur_gpt->control = GPTCR_SWR;

(Note I define the structure with volatile )

Dennis

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

[Simon Glass]

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.)

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.

I have seen various comments about this topic on the list, but I'm not
sure if there is a definitive document anywhere - you could write one
with U-Boot in mind if you like.

>
>
>   for example:
>
> struct base_gpt {
>         unsigned int control;
>         unsigned int prescaler;
> };
>
> #define GPT1_BASE_ADDR GPTCR_SWR
> volatile struct base_gpt *cur_gpt = (struct base_gpt *)GPT1_BASE_ADDR;
>
> A) acess with writel
>         __raw_writel(GPTCR_SWR, &cur_gpt->control);
>
> B) direct access
>        cur_gpt->control = GPTCR_SWR;
>
> (Note I define the structure with volatile )
>
> Dennis

Regards,
Simon

> _______________________________________________
> U-Boot mailing list
> U-Boot at lists.denx.de
> http://lists.denx.de/mailman/listinfo/u-boot

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

[Scott Wood]

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