linux-kernel - Re: [PATCH] platform/x86: intel_pmc_core: Report slp

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Date:   Fri, 5 Apr 2019 12:44:55 +0530
From:   Rajneesh Bhardwaj <rajneesh.bhardwaj@...el.com>
To:     Evan Green <evgreen@...omium.org>
Cc:     Rajat Jain <rajatja@...gle.com>,
        Furquan Shaikh <furquan@...omium.org>,
        Ravi Chandra Sadineni <ravisadineni@...omium.org>,
        Vishwanath Somayaji <vishwanath.somayaji@...el.com>,
        Andy Shevchenko <andy@...radead.org>,
        linux-kernel@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        Darren Hart <dvhart@...radead.org>
Subject: Re: [PATCH] platform/x86: intel_pmc_core: Report slp_s0 residency
 range

On Mon, Apr 01, 2019 at 11:05:04AM -0700, Evan Green wrote:
> The PMC driver performs a 32-bit read on the sleep s0 residency counter,
> followed by a hard-coded multiplication to convert into microseconds.
> The maximum value this counter could have would be 0xffffffff*0x64
> microseconds, which by my calculations is about 4.9 days. This is well
> within a reasonable time period to observe an overflow.
> 
> Usermode consumers watching slp_s0_residency_usec need to be aware of
> overflows, but have no idea what the maximum value of this counter is,
> given the hardcoded multiply of a 32-bit value by
> SPT_PMC_SLP_S0_RES_COUNTER_STEP.

This register is a 32 bit register untill ICL generation and a recent patch
from Rajat fixed the overflow https://patchwork.kernel.org/patch/10816103/
already so i am not sure how this will help userspace. I think the userspace
can still take care of any overflow concerns based on the information
available about this register in EDS so i feel exposing a new debugfs entry
just for the sake of knowing range is probably not needed.

> 
> Expose a slp_s0_residency_usec_range to usermode as well, which returns
> the maximum value this counter could have. Consumers can use this to
> manage rollovers.
> 
> Signed-off-by: Evan Green <evgreen@...omium.org>
> 
> ---
> 
> Note: I also looked at a similar bit of functionality in
> intel_pmc_s0ix_counter_read(), but noticed it's doing a 64-bit register
> access. Is the counter being read here in pmc_core_dev_state_get()
> (weird name btw) actually 64-bits long? If so, we can abandon this change
> and just create a fix to return the full extended value.
> 
> ---
>  drivers/platform/x86/intel_pmc_core.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/drivers/platform/x86/intel_pmc_core.c b/drivers/platform/x86/intel_pmc_core.c
> index f2c621b55f49..bec54be9be93 100644
> --- a/drivers/platform/x86/intel_pmc_core.c
> +++ b/drivers/platform/x86/intel_pmc_core.c
> @@ -396,6 +396,14 @@ static int pmc_core_dev_state_get(void *data, u64 *val)
>  
>  DEFINE_DEBUGFS_ATTRIBUTE(pmc_core_dev_state, pmc_core_dev_state_get, NULL, "%llu\n");
>  
> +static int pmc_core_slp_s0_range_get(void *data, u64 *val)
> +{
> +	*val = pmc_core_adjust_slp_s0_step(0xffffffff);
> +	return 0;
> +}
> +
> +DEFINE_DEBUGFS_ATTRIBUTE(pmc_core_slp_s0_range, pmc_core_slp_s0_range_get, NULL, "%llu\n");
> +
>  static int pmc_core_check_read_lock_bit(void)
>  {
>  	struct pmc_dev *pmcdev = &pmc;
> @@ -764,6 +772,9 @@ static int pmc_core_dbgfs_register(struct pmc_dev *pmcdev)
>  	debugfs_create_file("slp_s0_residency_usec", 0444, dir, pmcdev,
>  			    &pmc_core_dev_state);
>  
> +	debugfs_create_file("slp_s0_residency_usec_range", 0444, dir, pmcdev,
> +			    &pmc_core_slp_s0_range);
> +
>  	debugfs_create_file("pch_ip_power_gating_status", 0444, dir, pmcdev,
>  			    &pmc_core_ppfear_fops);
>  
> -- 
> 2.20.1
> 

-- 
Best Regards,
Rajneesh