linux-kernel - Re: [PATCH 01/10] coresight: trbe: Add infrastructure for Errata handling

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Message-ID: <4c23e288-14bd-f4a5-2f92-6e3ad46324fa@arm.com>
Date:   Mon, 2 Aug 2021 12:13:45 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, coresight@...ts.linaro.org,
        will@...nel.org, catalin.marinas@....com, james.morse@....com,
        mathieu.poirier@...aro.org, mike.leach@...aro.org,
        leo.yan@...aro.org, maz@...nel.org, mark.rutland@....com
Subject: Re: [PATCH 01/10] coresight: trbe: Add infrastructure for Errata
 handling



On 7/28/21 7:22 PM, Suzuki K Poulose wrote:
> Add a minimal infrastructure to keep track of the errata
> affecting the given TRBE instance. Given that we have
> heterogeneous CPUs, we have to manage the list per-TRBE
> instance to be able to apply the work around as needed.
> 
> We rely on the arm64 errata framework for the actual
> description and the discovery of a given erratum, to
> keep the Erratum work around at a central place and
> benefit from the code and the advertisement from the
> kernel. We use a local mapping of the erratum to
> avoid bloating up the individual TRBE structures.

I guess there is no other way around apart from each TRBE instance
tracking applicable erratas locally per CPU, even though it sounds
bit redundant.

> i.e, each arm64 TRBE erratum bit is assigned a new number
> within the driver to track. Each trbe instance updates
> the list of affected erratum at probe time on the CPU.
> This makes sure that we can easily access the list of
> errata on a given TRBE instance without much overhead.

It also ensures that the generic errata framework is queried just
once during individual CPU probe.

> 
> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
> Cc: Mike Leach <mike.leach@...aro.org>
> Cc: Leo Yan <leo.yan@...aro.org>
> Cc: Anshuman Khandual <anshuman.khandual@....com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> ---
>  drivers/hwtracing/coresight/coresight-trbe.c | 48 ++++++++++++++++++++
>  1 file changed, 48 insertions(+)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
> index b8586c170889..0368bf405e35 100644
> --- a/drivers/hwtracing/coresight/coresight-trbe.c
> +++ b/drivers/hwtracing/coresight/coresight-trbe.c
> @@ -16,6 +16,8 @@
>  #define pr_fmt(fmt) DRVNAME ": " fmt
>  
>  #include <asm/barrier.h>
> +#include <asm/cputype.h>
> +
>  #include "coresight-self-hosted-trace.h"
>  #include "coresight-trbe.h"
>  
> @@ -65,6 +67,35 @@ struct trbe_buf {
>  	struct trbe_cpudata *cpudata;
>  };
>  
> +/*
> + * TRBE erratum list
> + *
> + * We rely on the corresponding cpucaps to be defined for a given
> + * TRBE erratum. We map the given cpucap into a TRBE internal number
> + * to make the tracking of the errata lean.
> + *
> + * This helps in :
> + *   - Not duplicating the detection logic
> + *   - Streamlined detection of erratum across the system
> + *
> + * Since the erratum work arounds could be applied individually
> + * per TRBE instance, we keep track of the list of errata that
> + * affects the given instance of the TRBE.
> + */
> +#define TRBE_ERRATA_MAX			0
> +
> +static unsigned long trbe_errata_cpucaps[TRBE_ERRATA_MAX] = {
> +};

This needs to be tighten up. There should be build time guard rails in
arm64 errata cpucaps, so that only TRBE specific ones could be assigned
here as trbe_errata_cpucaps[].

> +
> +/*
> + * struct trbe_cpudata: TRBE instance specific data
> + * @trbe_flag		- TRBE dirty/access flag support
> + * @tbre_align		- Actual TRBE alignment required for TRBPTR_EL1.
> + * @cpu			- CPU this TRBE belongs to.
> + * @mode		- Mode of current operation. (perf/disabled)
> + * @drvdata		- TRBE specific drvdata
> + * @errata		- Bit map for the errata on this TRBE.
> + */
>  struct trbe_cpudata {
>  	bool trbe_flag;
>  	u64 trbe_align;
> @@ -72,6 +103,7 @@ struct trbe_cpudata {
>  	enum cs_mode mode;
>  	struct trbe_buf *buf;
>  	struct trbe_drvdata *drvdata;
> +	DECLARE_BITMAP(errata, TRBE_ERRATA_MAX);
>  };
>  
>  struct trbe_drvdata {
> @@ -84,6 +116,21 @@ struct trbe_drvdata {
>  	struct platform_device *pdev;
>  };
>  
> +static void trbe_check_errata(struct trbe_cpudata *cpudata)
> +{
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(trbe_errata_cpucaps); i++) {

BUILD_BUG_ON() - if trbe_errata_cpucaps[i] is not inside TRBE specific
errata cpucap range ?

> +		if (this_cpu_has_cap(trbe_errata_cpucaps[i]))
> +			set_bit(i, cpudata->errata);
> +	}
> +}
> +
> +static inline bool trbe_has_erratum(int i, struct trbe_cpudata *cpudata)

Switch the argument positions here ? 'int i' should be the second one.

> +{
> +	return (i < TRBE_ERRATA_MAX) && test_bit(i, cpudata->errata);
> +}
> +
>  static int trbe_alloc_node(struct perf_event *event)
>  {
>  	if (event->cpu == -1)
> @@ -925,6 +972,7 @@ static void arm_trbe_probe_cpu(void *info)
>  		goto cpu_clear;
>  	}
>  
> +	trbe_check_errata(cpudata);

This should be called right at the end before arm_trbe_probe_cpu() exits
on the success path. Errata should not be evaluated if TRBE on the CPU
wont be used for some reason i.e cpumask_clear_cpu() path.

>  	cpudata->trbe_align = 1ULL << get_trbe_address_align(trbidr);
>  	if (cpudata->trbe_align > SZ_2K) {
>  		pr_err("Unsupported alignment on cpu %d\n", cpu);
> 

This patch should be moved after [PATCH 5/10] i.e just before adding the
first TRBE errata.