Re: [PATCH v2] coresight: etm4x: Fix crash observed on Qcom ETM parts with 'Low power override'

[Leo Yan <leo.yan@linaro.org>]

Hi Bhupesh,

On Thu, Aug 25, 2022 at 10:52:32AM +0530, Bhupesh Sharma wrote:
> Some Qualcomm ETM implementations require skipping powering up
> the trace unit, as the ETMs are in the same power domain as
> their CPU cores.
> 
> Via commit 5214b563588e ("coresight: etm4x: Add support for
> sysreg only devices"), the setting of 'skip_power_up' flag was
> moved after the 'etm4_init_arch_data' function is called, whereas
> the flag value is itself used inside the function. This causes
> a crash when ETM mode 'Low-power state behavior override' is set
> on some Qualcomm parts.
> 
> Fix the same.
> 
> Fixes: 5214b563588e ("coresight: etm4x: Add support for sysreg only devices")
> Cc: Mike Leach <mike.leach@linaro.org>
> Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
> Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Bhupesh Sharma <bhupesh.sharma@linaro.org>
> ---
>  - v1 can be seen here: https://lore.kernel.org/lkml/20220803191236.3037591-1-bhupesh.sharma@linaro.org/
>  - Addressed the review comments from Suzuki.
>  - Rebased on linux-next.
> 
>  drivers/hwtracing/coresight/coresight-etm4x-core.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> index d39660a3e50c..14c1c7869795 100644
> --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
> +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> @@ -977,6 +977,16 @@ static bool etm4_init_sysreg_access(struct etmv4_drvdata *drvdata,
>  	if (!cpu_supports_sysreg_trace())
>  		return false;
>  
> +	/*
> +	 * Some Qualcomm implementations require skipping powering up the trace unit,
> +	 * as the ETMs are in the same power domain as their CPU cores.
> +	 *
> +	 * Since the 'skip_power_up' flag is used inside 'etm4_init_arch_data' function,
> +	 * initialize it before the function is called.
> +	 */
> +	if (fwnode_property_present(dev_fwnode(dev), "qcom,skip-power-up"))
> +		drvdata->skip_power_up = true;
> +

I personally think this sentence should be placed in the function
etm4_probe(), you need to move it just before smp call
etm4_init_arch_data(), this can allow DT property "qcom,skip-power-up"
to be respected.

>  	/*
>  	 * ETMs implementing sysreg access must implement TRCDEVARCH.
>  	 */
> @@ -1951,8 +1961,7 @@ static int etm4_probe(struct device *dev, void __iomem *base, u32 etm_pid)
>  		return -EINVAL;
>  
>  	/* TRCPDCR is not accessible with system instructions. */
> -	if (!desc.access.io_mem ||
> -	    fwnode_property_present(dev_fwnode(dev), "qcom,skip-power-up"))
> +	if (!desc.access.io_mem)
>  		drvdata->skip_power_up = true;

I prefer to move the condition checking for "desc.access.io_mem" to
etm4_init_sysreg_access(), this can make sure the flag skip_power_up
is set correctly based on property of system register access.

A side topic, in the mainline kernel I found the value
"desc.access.io_mem" is always zero (see the initialized value in
etm4_probe() and etm4_init_sysreg_access()).  Should we initialize
desc.access.io_mem to true in etm4_probe()?

diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
index d39660a3e50c..cf2555c50abb 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
+++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
@@ -1939,6 +1939,7 @@ static int etm4_probe(struct device *dev, void __iomem *base, u32 etm_pid)
        if (drvdata->cpu < 0)
                return drvdata->cpu;
 
+       desc.access.io_mem = true;
        init_arg.drvdata = drvdata;
        init_arg.csa = &desc.access;
        init_arg.pid = etm_pid;
  
Thanks,
Leo

>  	major = ETM_ARCH_MAJOR_VERSION(drvdata->arch);
> -- 
> 2.35.3
>