Re: [PATCH v2 1/6] MIPS: perf: More robustly probe for the presence of per-tc counters

[James Hogan <jhogan@kernel.org>]

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On Thu, Apr 12, 2018 at 10:36:21AM +0100, Matt Redfearn wrote:
> Processors implementing the MIPS MT ASE may have performance counters
> implemented per core or per TC. Processors implemented by MIPS
> Technologies signify presence per TC through a bit in the implementation
> specific Config7 register. Currently the code which probes for their
> presence blindly reads a magic number corresponding to this bit, despite
> it potentially having a different meaning in the CPU implementation.
> 
> The test of Config7.PTC was previously enabled when CONFIG_BMIPS5000 was
> enabled. However, according to [florian], the BMIPS5000 manual does not
> define this bit, so we can assume it is 0 and the feature is not
> supported.
> 
> Introduce probe_mipsmt_pertccounters() to probe for the presence of per
> TC counters. This detects the ases implemented in the CPU, and reads any
> implementation specific bit flagging their presence. In the case of MIPS
> implementations, this bit is Config7.PTC. A definition of this bit is
> added in mipsregs.h for MIPS Technologies. No other implementations
> support this feature.
> 
> Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
> ---
> 
> Changes in v2: None
> 
>  arch/mips/include/asm/mipsregs.h     |  5 +++++
>  arch/mips/kernel/perf_event_mipsxx.c | 29 ++++++++++++++++++++++++++++-
>  2 files changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/mips/include/asm/mipsregs.h b/arch/mips/include/asm/mipsregs.h
> index 858752dac337..a4baaaa02bc8 100644
> --- a/arch/mips/include/asm/mipsregs.h
> +++ b/arch/mips/include/asm/mipsregs.h
> @@ -684,6 +684,11 @@
>  #define MIPS_CONF7_IAR		(_ULCAST_(1) << 10)
>  #define MIPS_CONF7_AR		(_ULCAST_(1) << 16)
>  
> +/* Config7 Bits specific to MIPS Technologies. */
> +
> +/* Performance counters implemented Per TC */
> +#define MTI_CONF7_PTC		(_ULCAST_(1) << 19)
> +
>  /* WatchLo* register definitions */
>  #define MIPS_WATCHLO_IRW	(_ULCAST_(0x7) << 0)
>  
> diff --git a/arch/mips/kernel/perf_event_mipsxx.c b/arch/mips/kernel/perf_event_mipsxx.c
> index 6668f67a61c3..f3ec4a36921d 100644
> --- a/arch/mips/kernel/perf_event_mipsxx.c
> +++ b/arch/mips/kernel/perf_event_mipsxx.c
> @@ -1708,6 +1708,33 @@ static const struct mips_perf_event *xlp_pmu_map_raw_event(u64 config)
>  	return &raw_event;
>  }
>  
> +#ifdef CONFIG_MIPS_PERF_SHARED_TC_COUNTERS
> +/*
> + * The MIPS MT ASE specifies that performance counters may be implemented
> + * per core or per TC. If implemented per TC then all Linux CPUs have their
> + * own unique counters. If implemented per core, then VPEs in the core must
> + * treat the counters as a shared resource.
> + * Probe for the presence of per-TC counters
> + */
> +static int probe_mipsmt_pertccounters(void)
> +{
> +	struct cpuinfo_mips *c = &current_cpu_data;
> +
> +	/* Non-MT cores by definition cannot implement per-TC counters */
> +	if (!cpu_has_mipsmt)
> +		return 0;
> +
> +	switch (c->processor_id & PRID_COMP_MASK) {
> +	case PRID_COMP_MIPS:
> +		/* MTI implementations use CONFIG7.PTC to signify presence */
> +		return read_c0_config7() & MTI_CONF7_PTC;
> +	default:
> +		break;
> +	}
> +	return 0;
> +}
> +#endif /* CONFIG_MIPS_PERF_SHARED_TC_COUNTERS */
> +
>  static int __init
>  init_hw_perf_events(void)
>  {
> @@ -1723,7 +1750,7 @@ init_hw_perf_events(void)
>  	}
>  
>  #ifdef CONFIG_MIPS_PERF_SHARED_TC_COUNTERS
> -	cpu_has_mipsmt_pertccounters = read_c0_config7() & (1<<19);
> +	cpu_has_mipsmt_pertccounters = probe_mipsmt_pertccounters();

Similar code also exists in arch/mips/oprofile/op_model_mipsxx.c.
Perhaps it is time to unify it into more standard places, i.e.
asm/cpu-features.h and cpu-probe.c.

Cheers
James

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