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Message-ID: <c3804290-bdb1-d1eb-3526-9b0ce4c8e8b1@arm.com>
Date: Fri, 28 Jan 2022 09:08:20 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Rob Herring <robh@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
linux-perf-users@...r.kernel.org
Subject: Re: [RFC V1 03/11] arm64/perf: Update struct arm_pmu for BRBE
On 1/26/22 10:29 PM, Rob Herring wrote:
> On Mon, Jan 24, 2022 at 10:00:45AM +0530, Anshuman Khandual wrote:
>> This updates struct arm_pmu to include all required helpers that will drive
>
>>>From submitting-patches.rst:
>
> Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
> instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
> to do frotz", as if you are giving orders to the codebase to change
> its behaviour.
s/This updates struct arm_pmu to/Update struct arm_pmu to/ should address
this problem I guess. Although the difference here seems very subtle :)
>
>> BRBE functionality for a given PMU implementation. These are the following.
>
> Don't describe what the change is, one can read the diff for that.
Sure, though was trying just to enumerate a potential driver's functional
responsibilities.
> Answer why it is needed.
Sure.
>
> One thing to answer in the commit msg is why we need the hooks here.
> Have we concluded that adding BRBE hooks to struct arm_pmu for what is
> an armv8 specific feature is the right approach? I don't recall
> reaching that conclusion.
Although it might be possible to have this implementation embedded in
the existing armv8 PMU implementation, I still believe that the BRBE
functionalities abstracted out at the arm_pmu level with a separate
config option is cleaner, easier to follow and to maintain as well.
Besides some helpers i.e brbe_supported(), brbe_probe() and brbe_reset()
might not fit seamlessly, when tried to be embedded via existing arm_pmu
helpers in the armv8 implementation.
Nonetheless if arm_pmu based additional BRBE helpers is absolutely a no
go for folks here in general, will explore arm64 based implementation.
>
>>
>> - brbe_filter : Convert perf event filters into BRBE HW filters
>> - brbe_probe : Probe BRBE HW and capture its attributes
>> - brbe_enable : Enable BRBE HW with a given config
>> - brbe_disable : Disable BRBE HW
>> - brbe_read : Read BRBE buffer for captured branch records
>> - brbe_reset : Reset BRBE buffer
>> - brbe_supported: Whether BRBE is supported or not
>
> The function names seem pretty self-explanatory, but the text is needed,
> shouldn't it be in the struct declaration.
Sure, sounds good. Will move there.
>
> I'm not really a fan of patches adding dead code. That's not any less to
> review. Restructuring with 'no functional change' OTOH is helpful in
> reviewing.
Restructuring without functional change clarifies the purpose better.
Besides this is more relevant in this context where the functions will
be provided by a separate (and also detached) driver implementation.
>> A BRBE driver implementation needs to provide these functionalities.
>>
>> Cc: Will Deacon <will@...nel.org>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: linux-arm-kernel@...ts.infradead.org
>> Cc: linux-perf-users@...r.kernel.org
>> Cc: linux-kernel@...r.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>> ---
>> arch/arm64/kernel/perf_event.c | 36 ++++++++++++++++++++++++++++++++++
>> include/linux/perf/arm_pmu.h | 7 +++++++
>> 2 files changed, 43 insertions(+)
>>
>> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
>> index cab678ed6618..f6a47036b0b4 100644
>> --- a/arch/arm64/kernel/perf_event.c
>> +++ b/arch/arm64/kernel/perf_event.c
>> @@ -1015,6 +1015,35 @@ static int armv8pmu_filter_match(struct perf_event *event)
>> return evtype != ARMV8_PMUV3_PERFCTR_CHAIN;
>> }
>>
>> +static void armv8pmu_brbe_filter(struct pmu_hw_events *hw_event, struct perf_event *event)
>> +{
>> +}
>> +
>> +static void armv8pmu_brbe_enable(struct pmu_hw_events *hw_event)
>> +{
>> +}
>> +
>> +static void armv8pmu_brbe_disable(struct pmu_hw_events *hw_event)
>> +{
>> +}
>> +
>> +static void armv8pmu_brbe_read(struct pmu_hw_events *hw_event, struct perf_event *event)
>> +{
>> +}
>> +
>> +static void armv8pmu_brbe_probe(struct pmu_hw_events *hw_event)
>> +{
>> +}
>> +
>> +static void armv8pmu_brbe_reset(struct pmu_hw_events *hw_event)
>> +{
>> +}
>> +
>> +static bool armv8pmu_brbe_supported(struct perf_event *event)
>> +{
>> + return false;
>> +}
>> +
>> static void armv8pmu_reset(void *info)
>> {
>> struct arm_pmu *cpu_pmu = (struct arm_pmu *)info;
>> @@ -1247,6 +1276,13 @@ static int armv8_pmu_init(struct arm_pmu *cpu_pmu, char *name,
>>
>> cpu_pmu->pmu.event_idx = armv8pmu_user_event_idx;
>>
>> + cpu_pmu->brbe_filter = armv8pmu_brbe_filter;
>> + cpu_pmu->brbe_enable = armv8pmu_brbe_enable;
>> + cpu_pmu->brbe_disable = armv8pmu_brbe_disable;
>> + cpu_pmu->brbe_read = armv8pmu_brbe_read;
>> + cpu_pmu->brbe_probe = armv8pmu_brbe_probe;
>> + cpu_pmu->brbe_reset = armv8pmu_brbe_reset;
>> + cpu_pmu->brbe_supported = armv8pmu_brbe_supported;
>> cpu_pmu->name = name;
>> cpu_pmu->map_event = map_event;
>> cpu_pmu->attr_groups[ARMPMU_ATTR_GROUP_EVENTS] = events ?
>> diff --git a/include/linux/perf/arm_pmu.h b/include/linux/perf/arm_pmu.h
>> index 2512e2f9cd4e..c0dd0d6c5883 100644
>> --- a/include/linux/perf/arm_pmu.h
>> +++ b/include/linux/perf/arm_pmu.h
>> @@ -98,6 +98,13 @@ struct arm_pmu {
>> void (*reset)(void *);
>> int (*map_event)(struct perf_event *event);
>> int (*filter_match)(struct perf_event *event);
>> + void (*brbe_filter)(struct pmu_hw_events *hw_events, struct perf_event *event);
>> + void (*brbe_probe)(struct pmu_hw_events *hw_events);
>> + void (*brbe_enable)(struct pmu_hw_events *hw_events);
>> + void (*brbe_disable)(struct pmu_hw_events *hw_events);
>> + void (*brbe_read)(struct pmu_hw_events *hw_events, struct perf_event *event);
>> + void (*brbe_reset)(struct pmu_hw_events *hw_events);
>> + bool (*brbe_supported)(struct perf_event *event);
>> int num_events;
>> bool secure_access; /* 32-bit ARM only */
>> #define ARMV8_PMUV3_MAX_COMMON_EVENTS 0x40
>> --
>> 2.25.1
>>
>>
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