Re: question on memory barrier

["linux-os \(Dick Johnson\)" <linux-os@analogic.com>]

On Wed, 24 Aug 2005, Andy Isaacson wrote:

> On Wed, Aug 24, 2005 at 07:31:31PM +0200, moreau francis wrote:
>> --- "linux-os (Dick Johnson)" <linux-os@analogic.com> a écrit :
>>> On Wed, 24 Aug 2005, moreau francis wrote:
>>>> I'm currently trying to write a USB driver for Linux. The device must be
>>>> configured by writing some values into the same register but I want to be
>>>> sure that the writing order is respected by either the compiler and the
>>>> cpu.
> [snip]
>>>> static inline void dev_out(u32 *reg, u32 value)
>>>> {
>>>>        writel(value, regs);
>>>> }
>>>>
>>>> void config_dev(void)
>>>> {
>>>>        dev_out(reg_a, 0x0); /* first io */
>>>>        dev_out(reg_a, 0xA); /* second io */
>>>> }
>>>>
>>>
>>> This should be fine. The effect of the first bit of code
>>> plus all side-effects (if any) should be complete at the
>>> first effective sequence-point (;) but you need to
>
> While sequence points are relevant for purposes of reasoning about the
> effects of C code on the abstract state machine as defined by the C
> standard, they are irrelevant when talking about IO.
>
>> sorry but I'm not sure to understand you...Do you mean that the first write
>> into reg_a pointer will be completed before the second write because they're
>> separated by a (;) ?
>> Or because writes are encapsulated inside an inline function,
>> therefore compiler must execute every single writes before returning
>> from the inline function ?
>
> The first register write will be completed before the second register
> write because you use writel, which is defined to have the semantics you
> want.  (It uses a platform-specific method to guarantee this, possibly
> "volatile" or "asm("eieio")" or whatever method your platform requires.)
>
> The sequence points, by themselves, do not make any requirements on the
> externally visible behavior of the program.  Nor does the fact that
> there's an inline function involved.  It's just the writel() that
> contains the magic to force in-order execution.
>
> You might benefit by running your source code through gcc -E and seeing
> what the writel() expands to.  (I do something like "rm drivers/mydev.o;
> make V=1" and then copy-n-paste the gcc line, replacing the "-c -o mydev.o"
> options with -E.)
>
> The sequence point argument is obviously wrong, BTW - if it were the
> case that a mere sequence point required the compiler to have completed
> all externally-visible side effects, then almost every optimization that
> gcc does with -O2 would be impossible.  CSE, loop splitting, etc.
>
> -andy


Wrong. Reference:

http://www.phy.duke.edu/~rgb/General/c_book/c_book/chapter8/sequence_points.html

Cheers,
Dick Johnson
Penguin : Linux version 2.6.12.5 on an i686 machine (5537.79 BogoMips).
Warning : 98.36% of all statistics are fiction.

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