Re: [PATCH 3/8] perf/arm-cmn: Ensure dtm_idx is big enough

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Aug 19, 2024 at 04:00:06PM +0100, Robin Murphy wrote:
> On 16/08/2024 11:14 am, Mark Rutland wrote:
> > On Fri, Aug 09, 2024 at 08:15:42PM +0100, Robin Murphy wrote:
> > > While CMN_MAX_DIMENSION was bumped to 12 for CMN-650, that only supports
> > > up to a 10x10 mesh, so bumping dtm_idx to 256 bits at the time worked
> > > out OK in practice. However CMN-700 did finally support up to 144 XPs,
> > > and thus needs a worst-case 288 bits of dtm_idx for an aggregated XP
> > > event on a maxed-out config. Oops.
> > > 
> > > Fixes: 23760a014417 ("perf/arm-cmn: Add CMN-700 support")
> > > Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> > > ---
> > >   drivers/perf/arm-cmn.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/perf/arm-cmn.c b/drivers/perf/arm-cmn.c
> > > index 0e2e12e2f4fb..c9a2b21a7aec 100644
> > > --- a/drivers/perf/arm-cmn.c
> > > +++ b/drivers/perf/arm-cmn.c
> > > @@ -563,7 +563,7 @@ static void arm_cmn_debugfs_init(struct arm_cmn *cmn, int id) {}
> > >   struct arm_cmn_hw_event {
> > >   	struct arm_cmn_node *dn;
> > > -	u64 dtm_idx[4];
> > > +	u64 dtm_idx[5];
> > 
> > Can't we size this based on CMN_MAX_DIMENSION (or CMN_MAX_XPS or
> > CMN_MAX_DTMS), to make that clear?
> 
> Fair enough, I did go back and forth a little on that idea, but reached the
> opposite conclusion that documenting this with the assert to deliberately
> make it *not* look straightforward was nicer than wrestling with an accurate
> name for the logical quantity here, which strictly would be something like
> CMN_MAX_NODES_PER_TYPE_WE_CARE_ABOUT (there can already be up to 256 RN-Fs,
> but those aren't visible to the PMU).
> 
> I'll have another think on that approach - I do concur that the assert isn't
> *functionally* any better than automatically sizing the array.

FWIW, I'm happy with the value being an over-estimate, and with needing
a comment. What I'm really after is that the sizing of dtm_idx is clear
at the definition of dtm_idx, without an arbitrary-looking number.

Mark.