Subject: [PATCH 0/3] ARM sched_clock selection enhancements

From: [email-masked] (Rob Herring)
Date: Tue, 12 Mar 2013 07:43:58 -0500

On 03/12/2013 04:23 AM, Russell King - ARM Linux wrote:
> On Mon, Mar 11, 2013 at 09:26:34PM -0500, Rob Herring wrote:
>> In preparation to move more timer initialization to use CLKSRC_OF and
>> out of the platforms, a way to select the timer used for sched_clock is
>> needed. This series makes the ARM sched_clock support code prefer 64-bit
>> counters or higher frequency counters. This is sufficient at least on
>> ARM Ltd boards to use the 24MHz counter rather than sp804 and to always
>> use the 64-bit architected timer when present. This mechanism can be
>> extended to DT properties if needed for any non-discoverable h/w feature.
> 
> No - this stuff is designed specifically to cope with the dilemas of
> less-than-64-bit sched_clock sources.  If you have 64-bit sources, then
> that should be dealt with differently - this code will break with 64-bit
> sources due to the multiplication and shift losing the high order bits.

It is dealt with differently. In the 64-bit case, there is only a
multiply and all the code to deal with wrapping of 32-bit counters is
skipped. It is exactly the same conversion as arm64.

Rob