linux-kernel - Re: [PATCH] perf: CXL: fix mismatched number of counters mask

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Message-ID: <20230905145100.GC3322@willie-the-truck>
Date:   Tue, 5 Sep 2023 15:51:00 +0100
From:   Will Deacon <will@...nel.org>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     Jeongtae Park <jtp.park@...sung.com>, linux-cxl@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Kyungsan Kim <ks0204.kim@...sung.com>,
        Wonjae Lee <wj28.lee@...sung.com>,
        Hojin Nam <hj96.nam@...sung.com>,
        Junhyeok Im <junhyeok.im@...sung.com>,
        Jehoon Park <jehoon.park@...sung.com>
Subject: Re: [PATCH] perf: CXL: fix mismatched number of counters mask

On Tue, Sep 05, 2023 at 03:46:34PM +0100, Jonathan Cameron wrote:
> On Tue, 5 Sep 2023 15:28:54 +0100
> Will Deacon <will@...nel.org> wrote:
> 
> > On Tue, Sep 05, 2023 at 09:33:09PM +0900, Jeongtae Park wrote:
> > > The number of Count Units field is described as 6 bits long
> > > in the CXL 3.0 specification. However, its mask value was
> > > only declared as 5 bits long.
> > > 
> > > Signed-off-by: Jeongtae Park <jtp.park@...sung.com>
> > > ---
> > >  drivers/perf/cxl_pmu.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/perf/cxl_pmu.c b/drivers/perf/cxl_pmu.c
> > > index 0a8f597e695b..365d964b0f6a 100644
> > > --- a/drivers/perf/cxl_pmu.c
> > > +++ b/drivers/perf/cxl_pmu.c
> > > @@ -25,7 +25,7 @@
> > >  #include "../cxl/pmu.h"
> > >  
> > >  #define CXL_PMU_CAP_REG			0x0
> > > -#define   CXL_PMU_CAP_NUM_COUNTERS_MSK			GENMASK_ULL(4, 0)
> > > +#define   CXL_PMU_CAP_NUM_COUNTERS_MSK			GENMASK_ULL(5, 0)
> > >  #define   CXL_PMU_CAP_COUNTER_WIDTH_MSK			GENMASK_ULL(15, 8)
> > >  #define   CXL_PMU_CAP_NUM_EVN_CAP_REG_SUP_MSK		GENMASK_ULL(24, 20)
> > >  #define   CXL_PMU_CAP_FILTERS_SUP_MSK			GENMASK_ULL(39, 32)  
> > 
> > I don't have access to the CXL spec, but widening this mask looks like
> > it puts us out-of-whack with CXL_PMU_MAX_COUNTERS.
> > 
> > Did v3.0 of the spec bump the number of counters? If so, can you please
> > check that this is a backwards-compatible change?
> 
> CXL Performance monitors were only introduced in CXL 3.0 so not that.

Thanks for the information!

> The max value that register can take is 0x3f (0 based, so 64 counters ==
> CXL_PMU_MAX_COUNTERS)
> So it should be 6 bits wide. I did some history digging and this isn't
> even a draft spec / final spec issue - simple typo I guess.

Heh, I blame the heat as I thought this was extending to 7 bits for some
reason. Sorry about that.

> Fix seems correct to me.
> 
> Acked-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>

I'll pick it up, thanks again!

Will