Re: [PATCH V4 1/3] coresight: etm: Override TRCIDR3.CCITMIN on errata affected cpus

[Randy Dunlap]

Hi--

On 8/18/23 04:20, Anshuman Khandual wrote:
> This work arounds errata 1490853 on Cortex-A76, and Neoverse-N1, errata
> 1491015 on Cortex-A77, errata 1502854 on Cortex-X1, and errata 1619801 on
> Neoverse-V1, based affected cpus, where software read for TRCIDR3.CCITMIN
> field in ETM gets an wrong value.
> 
> If software uses the value returned by the TRCIDR3.CCITMIN register field,
> then it will limit the range which could be used for programming the ETM.
> In reality, the ETM could be programmed with a much smaller value than what
> is indicated by the TRCIDR3.CCITMIN field and still function correctly.
> 
> If software reads the TRCIDR3.CCITMIN register field, corresponding to the
> instruction trace counting minimum threshold, observe the value 0x100 or a
> minimum cycle count threshold of 256. The correct value should be 0x4 or a
> minimum cycle count threshold of 4.
> 
> This work arounds the problem via storing 4 in drvdata->ccitmin on affected
> systems where the TRCIDR3.CCITMIN has been 256, thus preserving cycle count
> threshold granularity.
> 
> These errata information has been updated in arch/arm64/silicon-errata.rst,
> but without their corresponding configs because these have been implemented
> directly in the driver.
> 
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> Cc: Mike Leach <mike.leach@xxxxxxxxxx>
> Cc: James Clark <james.clark@xxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Cc: coresight@xxxxxxxxxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
>  Documentation/arch/arm64/silicon-errata.rst   | 10 ++++++
>  .../coresight/coresight-etm4x-core.c          | 36 +++++++++++++++++++
>  2 files changed, 46 insertions(+)
> 

> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> index 7e307022303a..591fab73ee79 100644
> --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
> +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> @@ -1131,6 +1131,39 @@ static void cpu_detect_trace_filtering(struct etmv4_drvdata *drvdata)
>  	drvdata->trfcr = trfcr;
>  }
>  
> +/*
> + * The following errata on applicable cpu ranges, affect the CCITMIN filed
> + * in TCRIDR3 register. Software read for the field returns 0x100 limiting
> + * the cycle threshold granularity, where as the right value should have

                                       whereas

> + * been 0x4, which is well supported in the hardware.
> + */
> +static struct midr_range etm_wrong_ccitmin_cpus[] = {
> +	/* Erratum #1490853 - Cortex-A76 */
> +	MIDR_RANGE(MIDR_CORTEX_A76, 0, 0, 4, 0),
> +	/* Erratum #1490853 - Neoverse-N1 */
> +	MIDR_RANGE(MIDR_NEOVERSE_N1, 0, 0, 4, 0),
> +	/* Erratum #1491015 - Cortex-A77 */
> +	MIDR_RANGE(MIDR_CORTEX_A77, 0, 0, 1, 0),
> +	/* Erratum #1502854 - Cortex-X1 */
> +	MIDR_REV(MIDR_CORTEX_X1, 0, 0),
> +	/* Erratum #1619801 - Neoverse-V1 */
> +	MIDR_REV(MIDR_NEOVERSE_V1, 0, 0),
> +	{},
> +};
> +
> +static bool etm4_core_reads_wrong_ccitmin(struct etmv4_drvdata *drvdata)
> +{
> +	/*
> +	 * Erratum affected cpus will read 256 as the minimum
> +	 * instruction trace cycle counting threshold where as

	                                              whereas

> +	 * the correct value should be 4 instead. Override the
> +	 * recorded value for 'drvdata->ccitmin' to workaround
> +	 * this problem.
> +	 */

-- 
~Randy