On Fri, 3 Feb 2012, Russell King - ARM Linux wrote:

On Thu, Feb 02, 2012 at 03:36:49PM -0800, Stephen Boyd wrote:

quoted

On 02/02/12 13:38, Nicolas Pitre wrote:

quoted

What about a pair of helpers written in C instead?

v7_flush_dcache_all() could be renamed, and a wrapper function called 
v7_flush_dcache_all() would call the preemption disable helper, call the 
former v7_flush_dcache_all code, then call the preemption enable helper.

Then __v7_setup() could still call the core cache flush code without 
issues.

I tried to put the preemption disable/enable right around the place
where it was needed. With this approach we would disable preemption
during the entire cache flush. I'm not sure if we want to make this
function worse for performance, do we? It certainly sounds easier than
writing all the preempt macros in assembly though.

Err, why do you think it's a big task?

preempt disable is a case of incrementing the thread preempt count, while
preempt enable is a case of decrementing it, testing for zero, if zero,
then checking whether TIF_NEED_RESCHED is set and calling a function.

Oh certainly.  And we already do just that in a few places already.  I 
re-read your previous email to realize that I initially misread your 
remark about the ickness of explicitly calling the scheduler.

If that's too much, then the simple method in assembly to quickly disable
preemption over a very few set of instructions is using mrs/msr and cpsid i.
That'll be far cheaper than fiddling about with preempt counters or
messing about with veneers in C code.

Indeed.  And I think that would be plenty sufficient here as the 
protected region is really short.  I don't think that warrants any 
macros.


Nicolas