Re: [PATCH] ACPI/PPTT: Handle architecturally unknown cache types

[Sudeep Holla <sudeep.holla@arm.com>]

On Wed, Sep 12, 2018 at 09:57:14AM -0600, Jeffrey Hugo wrote:
> On 9/12/2018 9:38 AM, Sudeep Holla wrote:
> >
> >
> >On 12/09/18 16:27, Sudeep Holla wrote:
> >>
> >>
> >>On 12/09/18 15:41, Jeffrey Hugo wrote:
> >
> >[...]
> >
> >>>
> >>>Correct.  However, what if you have a NOCACHE (not architecturally
> >>>specified), that is fully described in PPTT, as a non-unified cache
> >>>(data only)?  Unlikely?  Maybe.  Still seem possible though, therefore I
> >>>feel this assumption is suspect.
> >>>
> >>
> >>Yes, we have other issues if the architecturally not specified cache is
> >>not unified irrespective of what PPTT says. So we may need to review and
> >>see if that assumption is removed everywhere.
> >>
> >>Until then why can't a simple change fix the issue you have:
> >>
> >>-->8
> >>
> >>diff --git i/drivers/acpi/pptt.c w/drivers/acpi/pptt.c
> >>index d1e26cb599bf..f74131201f5e 100644
> >>--- i/drivers/acpi/pptt.c
> >>+++ w/drivers/acpi/pptt.c
> >>@@ -406,7 +406,8 @@ static void update_cache_properties(struct cacheinfo
> >>*this_leaf,
> >>          * update the cache type as well.
> >>          */
> >>         if (this_leaf->type == CACHE_TYPE_NOCACHE &&
> >>-           valid_flags == PPTT_CHECKED_ATTRIBUTES)
> >>+           (valid_flags == PPTT_CHECKED_ATTRIBUTES ||
> >>+            found_cache->flags & ACPI_PPTT_CACHE_TYPE_VALID))
> >
> >Looking at this again, if we are supporting just presence of cache type
> >and size(may be), then we can drop the whole valid_flags thing here.
> >
> >>                 this_leaf->type = CACHE_TYPE_UNIFIED;
> >>  }
> >>
>
> Yes, this change fixes my usecase.  I think it invalidates the comment, and
> really, the comment should probably mention that we assume unified type
> because there are other issues in supporting architecturally not specified
> inst/data only caches.
>

Agreed.

> Do you want a V2 with this?  If so, do you want the fixes tag removed since
> you seem to view this as not a bug?
>

Yes please, I am fine to retain fixes tag but that's my opinion.

> I don't think I clearly understand the purpose of the valid flags, therefore
> I feel as though I'm not sure if it can be dropped or not.  Is it fair to
> say that what the valid flags is accomplishing is identifying if we have a
> sufficient level of information to support this cache?  If not, then should
> the cacheinfo driver not expose any sysfs information about the cache?
>

I don't see the use of the flag if we *have to* support the case where
all the cache geometry is not present but just cache type (and maybe
size?) is present. If that's the case we can drop valid flags entirely.
I really don't like the idea of supporting that, but I don't have strong
reasons to defend my idea, so I am fine with that.

Further, I think in your case with NOCACHE type set, sysfs dir shouldn't
have been created at the first place ideally. I think we need something
like below to fix that.

-->8

diff --git i/drivers/base/cacheinfo.c w/drivers/base/cacheinfo.c
index 5d5b5988e88b..cf78fa6d470d 100644
--- i/drivers/base/cacheinfo.c
+++ w/drivers/base/cacheinfo.c
@@ -615,6 +615,8 @@ static int cache_add_dev(unsigned int cpu)
 		this_leaf = this_cpu_ci->info_list + i;
 		if (this_leaf->disable_sysfs)
 			continue;
+		if (this_leaf->type == CACHE_TYPE_NOCACHE)
+			break;
 		cache_groups = cache_get_attribute_groups(this_leaf);
 		ci_dev = cpu_device_create(parent, this_leaf, cache_groups,
 					   "index%1u", i);