Re: [PATCH 3/4] MIPS: cpu-probe: Set the write-combine CCA value on per core basis

Hi Markos,

2014-08-12 0:23 GMT-07:00 Markos Chandras [off-list ref]:

Hi Florian,

On Mon, Aug 04, 2014 at 03:54:49PM -0700, Florian Fainelli wrote:

quoted

Hi Markos,

On 07/18/2014 02:51 AM, Markos Chandras wrote:

quoted

Different cores use different CCA values to achieve write-combine
memory writes. For cores that do not support write-combine we
set the default value to CCA:2 (uncached, non-coherent) which is the
default value as set by the kernel.

Signed-off-by: Markos Chandras <redacted>
---

[snip]
                      break;

quoted

@@ -765,67 +767,83 @@ static inline void cpu_probe_legacy(struct cpuinfo_mips *c, unsigned int cpu)

 static inline void cpu_probe_mips(struct cpuinfo_mips *c, unsigned int cpu)
 {
+   c->writecombine = _CACHE_UNCACHED_ACCELERATED;

Why do we set this writecombine setting by default, when later we are
going to override writecombine on a per-cpu basic.

In the end, we have the following:

cpu_probe()
      c->writecombine = _CACHE_UNCACHED;

      cpu_probe_mips()
              c->writecombine = _CACHE_UNCACHED_ACCELERATED:
              ... per-cpu case ...
              c->writecombine = _CACHE_UNCACHED;

Can't we just eliminate the various assignments in cpu_probe_mips() and
only override c->writecombine if _CACHE_UNCACHED is not suitable?

The reason I did it like this, is that new cores (eg *Aptiv family) will use
_CACHE_UNCACHED_ACCELERATED and that's why it's the 'default' option for
the MIPS cores. _CACHE_UNCACHED is only suitable for old cores.
The way it is right now, allows us to not have to set this option whenever we
add support for a new core since it will inherit the default option.

Ok, that makes sense, although we currently have more _CACHE_UNCACHED
platforms supported than _CACHE_UNCACHED_ACCELERATED, so maybe once
you reach a critical number of Aptiv cores or with similar caching
settings, we can reverse the tendency by then? This is not a strong
objection, I was just looking at a way to minimize the changes.
-- 
Florian