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Wed, 1 Jul 2026 18:56:25 +0800 (CST) Received: from [10.67.121.62] (10.67.121.62) by kwepemr200004.china.huawei.com (7.202.195.241) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.11; Wed, 1 Jul 2026 18:56:24 +0800 Message-ID: <737355ec-7e0c-434b-b41d-201d08d1d020@huawei.com> Date: Wed, 1 Jul 2026 18:56:23 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] arm64: topology: read CPPC FFH feedback counters in one operation To: Sumit Gupta , , , , , , , , , CC: , , , , , , , , , , , , "linux-tegra@vger.kernel.org" References: <20260410094145.4132082-1-zhangpengjie2@huawei.com> <20260410094145.4132082-3-zhangpengjie2@huawei.com> <56a1e8bf-41b1-40cf-a943-79cce84a774e@nvidia.com> From: Pengjie Zhang In-Reply-To: <56a1e8bf-41b1-40cf-a943-79cce84a774e@nvidia.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.67.121.62] X-ClientProxiedBy: kwepems100001.china.huawei.com (7.221.188.238) To kwepemr200004.china.huawei.com (7.202.195.241) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260701_035643_510497_3E5C7665 X-CRM114-Status: GOOD ( 17.74 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 6/29/2026 11:27 PM, Sumit Gupta wrote: > > On 10/04/26 15:11, Pengjie Zhang wrote: >> External email: Use caution opening links or attachments >> >> >> arm64 implements CPPC FFH feedback-counter reads using AMU counters. >> Because those counters must be sampled on the target CPU, reading the >> delivered and reference counters separately widens the observation >> window >> between them. >> >> Implement the paired FFH feedback-counter read hook on arm64 and sample >> both AMU counters together before decoding the requested CPC register >> values. >> >> Also factor the FFH bitfield extraction logic into a helper and reuse >> it from the existing single-counter FFH read path. >> >> Signed-off-by: Pengjie Zhang >> --- >>   arch/arm64/kernel/topology.c | 75 ++++++++++++++++++++++++++++++++---- >>   1 file changed, 67 insertions(+), 8 deletions(-) >> >> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c >> index b32f13358fbb..b90a767b2a1f 100644 >> --- a/arch/arm64/kernel/topology.c >> +++ b/arch/arm64/kernel/topology.c >> @@ -50,6 +50,16 @@ struct amu_cntr_sample { >>          unsigned long   last_scale_update; >>   }; >> >> +struct amu_ffh_ctrs { >> +       u64 corecnt; >> +       u64 constcnt; >> +}; >> + >> +enum cpc_ffh_ctr_id { >> +       CPC_FFH_CTR_CORE  = 0x0, >> +       CPC_FFH_CTR_CONST = 0x1, >> +}; >> + >>   static DEFINE_PER_CPU_SHARED_ALIGNED(struct amu_cntr_sample, >> cpu_amu_samples); >> >>   void update_freq_counters_refs(void) >> @@ -397,7 +407,7 @@ static void cpu_read_constcnt(void *val) >>   } >> >>   static inline >> -int counters_read_on_cpu(int cpu, smp_call_func_t func, u64 *val) >> +int counters_read_on_cpu(int cpu, smp_call_func_t func, void *val) >>   { >>          /* >>           * Abort call on counterless CPU. >> @@ -447,24 +457,73 @@ bool cpc_ffh_supported(void) >>          return true; >>   } >> >> +static void amu_read_core_const_ctrs(void *val) >> +{ >> +       struct amu_ffh_ctrs *ctrs = val; >> + >> +       cpu_read_constcnt(&ctrs->constcnt); >> +       cpu_read_corecnt(&ctrs->corecnt); >> +} > > Any reason to flip the order? > Harmless as they are read back to back, but better to add a comment > if it's intentional. > > Thanks, > Sumit > .... > Hi Sumit， Thanks for taking the time to review and test. The cpu_read_constcnt() function includes the conditional check this_cpu_has_cap(ARM64_WORKAROUND_2457168), which incurs a latency of 6–12 nanoseconds on the our platform. If cpu_read_corecnt() is called prior to cpu_read_constcnt(), it will widen the sampling interval between the corecnt and constcnt counter readings. To address this, we have adjusted the call order: cpu_read_constcnt() is executed first, followed by cpu_read_corecnt(). I will add comments later. Thanks,   Pengjie > >