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Subject: Re: [PATCH v5 09/15] coresight: trbe: Add infrastructure for Errata
 handling
To:     Suzuki K Poulose <suzuki.poulose@arm.com>, will@kernel.org,
        mathieu.poirier@linaro.org
Cc:     catalin.marinas@arm.com, mike.leach@linaro.org, leo.yan@linaro.org,
        maz@kernel.org, coresight@lists.linaro.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
References: <20211014223125.2605031-1-suzuki.poulose@arm.com>
 <20211014223125.2605031-10-suzuki.poulose@arm.com>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <addefe1c-64b1-6c80-f30d-54cddef2c4b3@arm.com>
Date:   Tue, 19 Oct 2021 10:55:36 +0530
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On 10/15/21 4:01 AM, 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.
> Thus we will need to check if individual CPUs are affected
> by the erratum.
> 
> 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. Though we could reuse the "this_cpu_has_cap()"
> to apply an erratum work around, it is a bit of a heavy
> operation, as it must go through the "erratum" detection
> check on the CPU every time it is called (e.g, scanning
> through a table of affected MIDRs). Since we need
> to do this check for every session, may be multiple
> times (depending on the wrok around), we could save
> the cycles by caching the affected errata per-CPU
> instance in the per-CPU struct trbe_cpudata.
> 
> Since we are only interested in the errata affecting
> the TRBE driver, we only need to track a very few of them
> per-CPU.  Thus we use a local mapping of the CPUCAP for the
> erratum to avoid bloating up a bitmap for trbe_cpudata.
> 
> i.e, each arm64 TRBE erratum bit is assigned a "index"
> 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.
> 
> Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
> Cc: Mike Leach <mike.leach@linaro.org>
> Cc: Leo Yan <leo.yan@linaro.org>
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
> Changes since v4:
>   - Ensure the arm_trbe_probe_cpu() is called from non preemptible
>     context for hotplugged CPUs

This (both renaming and non preemptible context) makes sense.

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>

A small nit though.

Should the array sentinel value '-1' be bit formalized with a
macro and check like this != instead.

if (WARN_ON_ONCE(cap != TRBE_ERRATA_SENTINEL))
 

> Changes since v2:
>   - Automatically define TRBE_ERRATA_MAX
>   - Add some basic sanity check to make sure the new entries
>     are added in order.
>   - Describe the design choice of caching CPU local errata
>     in trbe_cpudata instead of using this_cpu_has_cap()
> Changes since v1:
>   - Flip the order of args for trbe_has_erratum()
>   - Move erratum detection further down in the sequence
> ---
>  drivers/hwtracing/coresight/coresight-trbe.c | 71 +++++++++++++++++++-
>  1 file changed, 68 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
> index f8c04c428780..314e5e7374c7 100644
> --- a/drivers/hwtracing/coresight/coresight-trbe.c
> +++ b/drivers/hwtracing/coresight/coresight-trbe.c
> @@ -16,6 +16,7 @@
>  #define pr_fmt(fmt) DRVNAME ": " fmt
>  
>  #include <asm/barrier.h>
> +
>  #include "coresight-self-hosted-trace.h"
>  #include "coresight-trbe.h"
>  
> @@ -67,14 +68,43 @@ struct trbe_buf {
>  	struct trbe_cpudata *cpudata;
>  };
>  
> +/*
> + * TRBE erratum list
> + *
> + * The errata are defined in arm64 generic cpu_errata framework.
> + * Since the errata work arounds could be applied individually
> + * to the affected CPUs inside the TRBE driver, we need to know if
> + * a given CPU is affected by the erratum. Unlike the other erratum
> + * work arounds, TRBE driver needs to check multiple times during
> + * a trace session. Thus we need a quicker access to per-CPU
> + * errata and not issue costly this_cpu_has_cap() everytime.
> + * We keep a set of the affected errata in trbe_cpudata, per TRBE.
> + *
> + * 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
> + */
> +
> +static int trbe_errata_cpucaps[] = {
> +	-1,		/* Sentinel, must be the last entry */
> +};
> +
> +/* The total number of listed errata in trbe_errata_cpucaps */
> +#define TRBE_ERRATA_MAX			(ARRAY_SIZE(trbe_errata_cpucaps) - 1)
> +
>  /*
>   * struct trbe_cpudata: TRBE instance specific data
>   * @trbe_flag		- TRBE dirty/access flag support
>   * @trbe_hw_align	- Actual TRBE alignment required for TRBPTR_EL1.
> - * @trbe_align		- Software alignment used for the TRBPTR_EL1,
> + * @trbe_align		- Software alignment used for the 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;
> @@ -84,6 +114,7 @@ struct trbe_cpudata {
>  	enum cs_mode mode;
>  	struct trbe_buf *buf;
>  	struct trbe_drvdata *drvdata;
> +	DECLARE_BITMAP(errata, TRBE_ERRATA_MAX);
>  };
>  
>  struct trbe_drvdata {
> @@ -96,6 +127,25 @@ struct trbe_drvdata {
>  	struct platform_device *pdev;
>  };
>  
> +static void trbe_check_errata(struct trbe_cpudata *cpudata)
> +{
> +	int i;
> +
> +	for (i = 0; i < TRBE_ERRATA_MAX; i++) {
> +		int cap = trbe_errata_cpucaps[i];
> +
> +		if (WARN_ON_ONCE(cap < 0))
> +			return;
> +		if (this_cpu_has_cap(cap))
> +			set_bit(i, cpudata->errata);
> +	}
> +}
> +
> +static inline bool trbe_has_erratum(struct trbe_cpudata *cpudata, int i)
> +{
> +	return (i < TRBE_ERRATA_MAX) && test_bit(i, cpudata->errata);
> +}
> +
>  static int trbe_alloc_node(struct perf_event *event)
>  {
>  	if (event->cpu == -1)
> @@ -952,6 +1002,9 @@ static void arm_trbe_register_coresight_cpu(struct trbe_drvdata *drvdata, int cp
>  	cpumask_clear_cpu(cpu, &drvdata->supported_cpus);
>  }
>  
> +/*
> + * Must be called with preemption disabled, for trbe_check_errata().
> + */
>  static void arm_trbe_probe_cpu(void *info)
>  {
>  	struct trbe_drvdata *drvdata = info;
> @@ -979,6 +1032,12 @@ static void arm_trbe_probe_cpu(void *info)
>  		goto cpu_clear;
>  	}
>  
> +	/*
> +	 * Run the TRBE erratum checks, now that we know
> +	 * this instance is about to be registered.
> +	 */
> +	trbe_check_errata(cpudata);
> +
>  	cpudata->trbe_align = cpudata->trbe_hw_align;
>  	cpudata->trbe_flag = get_trbe_flag_update(trbidr);
>  	cpudata->cpu = cpu;
> @@ -1032,18 +1091,24 @@ static int arm_trbe_remove_coresight(struct trbe_drvdata *drvdata)
>  	return 0;
>  }
>  
> +static void arm_trbe_probe_hotplugged_cpu(struct trbe_drvdata *drvdata)
> +{
> +	preempt_disable();
> +	arm_trbe_probe_cpu(drvdata);
> +	preempt_enable();
> +}
> +
>  static int arm_trbe_cpu_startup(unsigned int cpu, struct hlist_node *node)
>  {
>  	struct trbe_drvdata *drvdata = hlist_entry_safe(node, struct trbe_drvdata, hotplug_node);
>  
>  	if (cpumask_test_cpu(cpu, &drvdata->supported_cpus)) {
> -
>  		/*
>  		 * If this CPU was not probed for TRBE,
>  		 * initialize it now.
>  		 */
>  		if (!coresight_get_percpu_sink(cpu)) {
> -			arm_trbe_probe_cpu(drvdata);
> +			arm_trbe_probe_hotplugged_cpu(drvdata);
>  			if (cpumask_test_cpu(cpu, &drvdata->supported_cpus))
>  				arm_trbe_register_coresight_cpu(drvdata, cpu);
>  			if (cpumask_test_cpu(cpu, &drvdata->supported_cpus))
>