linux-kernel - Re: [PATCH 3/8] perf/arm-cmn: Ensure dtm

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Message-ID: <22b69400-af16-4e78-8f72-c10564d7cc6c@arm.com>
Date: Mon, 19 Aug 2024 16:00:06 +0100
From: Robin Murphy <robin.murphy@....com>
To: Mark Rutland <mark.rutland@....com>
Cc: will@...nel.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, ilkka@...amperecomputing.com
Subject: Re: [PATCH 3/8] perf/arm-cmn: Ensure dtm_idx is big enough

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@....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.

Thanks,
Robin.

>  From the desciription in the commit message it sounds like you need 2 *
> CMN_MAX_XPS bits, i.e.
> 
> 	#define DTM_IDX_BITS	(2 * CMN_MAX_XPS)
> 
> 	u64 dtm_idx[DIV_ROUND_UP(DTM_IDX_BITS, 64)];
> 
> Mark.
> 
>>   	s8 dtc_idx[CMN_MAX_DTCS];
>>   	u8 num_dns;
>>   	u8 dtm_offset;
>> -- 
>> 2.39.2.101.g768bb238c484.dirty
>>