[PATCH] use CLOCK_MONOTONIC_COARSE posix timer instead of CLOCK_MONOTONIC in Xorg

[Daniel Stone]

On Tue, Aug 24, 2010 at 03:55:41PM +0800, ykzhao wrote:
> On Tue, 2010-08-24 at 09:59 +0800, Daniel Stone wrote:
> > That doesn't change anything - if a system is using ID 6 for something
> > else, then using 6 is wholly incorrect, no matter whether you use the
> > constant directly, define some other symbol for it, or whatever.
> 
> Maybe the system is using the ID 6 for something else. But in theory it
> should not affect the system-call of posix timer.  

What do you mean, it shouldn't affect anything? What if timer ID 6 is
being used for 'number of seconds since last email to xorg-devel'?

> If the CLOCK_MONOTONIC_COARSE posix timer is already supported in the
> linux kernel(clockid is 6),  the system-call of "clock_getres/gettime(6,
> &tp)" will return the valid value.

It will return a valid value _for that clock_.  That clock may be
exactly what you don't want, if clock ID 6 is actually something else.

> If the CLOCK_MONOTONIC_COARSE is not supported in the linux kernel, the
> system-call of "clock_getres/gettime(6, &tp)" will return the invalid
> value, which indicates that we will fallback to the next posix
> timer(CLOCK_MONOTONIC). 

Hopefully, yes.

> Of course there exists another scenario. The CLOCK_MONOTONIC_COARSE
> posix timer is not supported. In course of compiling the Xorg we add the
> macro definition of "CLOCK_MONOTONIC_COARSE=3".

3?!?

> In such case we will use
> the incorrect posix timer after calling the function of
> clock_getres(CLOCK_MONOTONIC_COARSE, &tp). But if we compile it again
> under the latest glibc, we will get the warning message of "macro
> redefinition". 
> 
> Based on the above consideration, not sure whether it is meaningful to
> use the constant value(6) for the system-call of clock_getres? Otherwise
> we will have to consider the following two cases:
>     a. CLOCK_MONOTONIC_COARSE is not defined
> in /usr/include/linux/time.h
>     b. CLOCK_MONOTONIC_COARSE is already defined. But the corresponding
> id is not 6.
> 
> If wrong, please correct me. 

6 is not meaningful in any way.  Only CLOCK_MONOTONIC_COARSE, _as
defined by the system headers_, means a coarse/non-HPET monotonic clock.
Nothing else - not 6, not 3.  So, if it's not in the system headers, I
don't think we should be using it.  I'm not really sure if I can be more
clear ...
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