linux-kernel - Re: [PATCH v2 1/1] coresight: Fix TRCCONFIGR.QE sysfs interface

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Date:   Tue, 1 Feb 2022 10:00:23 -0700
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     James Clark <james.clark@....com>
Cc:     coresight@...ts.linaro.org, mike.leach@...aro.org,
        suzuki.poulose@....com, leo.yan@...aro.com,
        Leo Yan <leo.yan@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] coresight: Fix TRCCONFIGR.QE sysfs interface

On Thu, Jan 20, 2022 at 11:30:47AM +0000, James Clark wrote:
> It's impossible to program a valid value for TRCCONFIGR.QE
> when TRCIDR0.QSUPP==0b10. In that case the following is true:
> 
>   Q element support is implemented, and only supports Q elements without
>   instruction counts. TRCCONFIGR.QE can only take the values 0b00 or 0b11.
> 
> Currently the low bit of QSUPP is checked to see if the low bit of QE can
> be written to, but as you can see when QSUPP==0b10 the low bit is cleared
> making it impossible to ever write the only valid value of 0b11 to QE.
> 0b10 would be written instead, which is a reserved QE value even for all
> values of QSUPP.
> 
> The fix is to allow writing the low bit of QE for any non zero value of
> QSUPP.
> 
> This change also ensures that the low bit is always set, even when the
> user attempts to only set the high bit.
> 
> Signed-off-by: James Clark <james.clark@....com>
> Reviewed-by: Mike Leach <mike.leach@...aro.org>
> ---
>  drivers/hwtracing/coresight/coresight-etm4x-sysfs.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-sysfs.c b/drivers/hwtracing/coresight/coresight-etm4x-sysfs.c
> index a0640fa5c55b..57e94424a8d6 100644
> --- a/drivers/hwtracing/coresight/coresight-etm4x-sysfs.c
> +++ b/drivers/hwtracing/coresight/coresight-etm4x-sysfs.c
> @@ -367,8 +367,12 @@ static ssize_t mode_store(struct device *dev,
>  	mode = ETM_MODE_QELEM(config->mode);
>  	/* start by clearing QE bits */
>  	config->cfg &= ~(BIT(13) | BIT(14));
> -	/* if supported, Q elements with instruction counts are enabled */
> -	if ((mode & BIT(0)) && (drvdata->q_support & BIT(0)))
> +	/*
> +	 * if supported, Q elements with instruction counts are enabled.
> +	 * Always set the low bit for any requested mode. Valid combos are
> +	 * 0b00, 0b01 and 0b11.
> +	 */
> +	if (mode && drvdata->q_support)
>  		config->cfg |= BIT(13);

Interesting brain gymnastic - applied.

Thanks,
Mathieu

>  	/*
>  	 * if supported, Q elements with and without instruction
> -- 
> 2.28.0
>