Re: [PATCH V2 5/7] coresight: trbe: Work around the ignored system register writes

[Suzuki K Poulose]

On 07/01/2022 01:10, Anshuman Khandual wrote:
> TRBE implementations affected by Arm erratum #2064142 might fail to write
> into certain system registers after the TRBE has been disabled. Under some
> conditions after TRBE has been disabled, writes into certain TRBE registers
> TRBLIMITR_EL1, TRBPTR_EL1, TRBBASER_EL1, TRBSR_EL1 and TRBTRG_EL1 will be
> ignored and not be effected.
>
> Work around this problem in the TRBE driver by executing TSB CSYNC and DSB
> just after the trace collection has stopped and before performing a system
> register write to one of the affected registers. This just updates the TRBE
> driver as required.
>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>
> Cc: Suzuki Poulose <suzuki.poulose@xxxxxxx>
> Cc: coresight@xxxxxxxxxxxxxxxx
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
>   arch/arm64/Kconfig                           |  2 +-
>   drivers/hwtracing/coresight/coresight-trbe.c | 54 ++++++++++++++------
>   drivers/hwtracing/coresight/coresight-trbe.h |  8 ---
>   3 files changed, 39 insertions(+), 25 deletions(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index f1651cb71ef3..b6d62672bf7d 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -780,7 +780,7 @@ config ARM64_ERRATUM_2224489
>   
>   config ARM64_ERRATUM_2064142
>   	bool "Cortex-A510: 2064142: workaround TRBE register writes while disabled"
> -	depends on COMPILE_TEST # Until the CoreSight TRBE driver changes are in
> +	depends on CORESIGHT_TRBE
>   	default y
>   	help
>   	  This option adds the workaround for ARM Cortex-A510 erratum 2064142.
> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
> index 276862c07e32..850e9fca6847 100644
> --- a/drivers/hwtracing/coresight/coresight-trbe.c
> +++ b/drivers/hwtracing/coresight/coresight-trbe.c
> @@ -91,10 +91,12 @@ struct trbe_buf {
>    */
>   #define TRBE_WORKAROUND_OVERWRITE_FILL_MODE	0
>   #define TRBE_WORKAROUND_WRITE_OUT_OF_RANGE	1
> +#define TRBE_NEEDS_DRAIN_AFTER_DISABLE		2
>   
>   static int trbe_errata_cpucaps[] = {
>   	[TRBE_WORKAROUND_OVERWRITE_FILL_MODE] = ARM64_WORKAROUND_TRBE_OVERWRITE_FILL_MODE,
>   	[TRBE_WORKAROUND_WRITE_OUT_OF_RANGE] = ARM64_WORKAROUND_TRBE_WRITE_OUT_OF_RANGE,
> +	[TRBE_NEEDS_DRAIN_AFTER_DISABLE] = ARM64_WORKAROUND_2064142,
>   	-1,		/* Sentinel, must be the last entry */
>   };
>   
> @@ -167,6 +169,11 @@ static inline bool trbe_may_write_out_of_range(struct trbe_cpudata *cpudata)
>   	return trbe_has_erratum(cpudata, TRBE_WORKAROUND_WRITE_OUT_OF_RANGE);
>   }
>   
> +static inline bool trbe_needs_drain_after_disable(struct trbe_cpudata *cpudata)
> +{
> +	return trbe_has_erratum(cpudata, TRBE_NEEDS_DRAIN_AFTER_DISABLE);
> +}
> +
>   static int trbe_alloc_node(struct perf_event *event)
>   {
>   	if (event->cpu == -1)
> @@ -174,30 +181,42 @@ static int trbe_alloc_node(struct perf_event *event)
>   	return cpu_to_node(event->cpu);
>   }
>   
> -static void trbe_drain_buffer(void)
> +static inline void trbe_drain_buffer(void)
>   {
>   	tsb_csync();
>   	dsb(nsh);
>   }
>   
> -static void trbe_drain_and_disable_local(void)
> +static inline void set_trbe_disabled(struct trbe_cpudata *cpudata)
>   {
>   	u64 trblimitr = read_sysreg_s(SYS_TRBLIMITR_EL1);
>   
> -	trbe_drain_buffer();
> -
>   	/*
>   	 * Disable the TRBE without clearing LIMITPTR which
>   	 * might be required for fetching the buffer limits.
>   	 */
>   	trblimitr &= ~TRBLIMITR_ENABLE;
>   	write_sysreg_s(trblimitr, SYS_TRBLIMITR_EL1);
> +
> +	/*
> +	 * Errata affected TRBE implementation will need TSB CSYNC and
> +	 * DSB in order to prevent subsequent writes into certain TRBE
> +	 * system registers from being ignored and not effected.
> +	 */

minor nit: This comment could be moved to the definition of the
function "trbe_needs_drain_after_disable()" to make more sense.
The name is implicit here indicating, why we are doing a drain.

Either ways:

Reviewed-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>