Re: [PATCH 1/1] coresight: Fix TRCCONFIGR.QE sysfs interface

From: Mike Leach <mike.leach@linaro.org>
To: James Clark <James.Clark@arm.com>
Cc: mathieu.poirier@linaro.org, coresight@lists.linaro.org,
	suzuki.poulose@arm.com, Leo Yan <leo.yan@linaro.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] coresight: Fix TRCCONFIGR.QE sysfs interface
Date: Wed, 19 Jan 2022 15:22:17 +0000	[thread overview]
Message-ID: <CAJ9a7VhedrLMKfXBqzD_9fFxzKTg63-9wUM38Tw6ukhmSR9aoA@mail.gmail.com> (raw)
In-Reply-To: <20220114173254.1876151-2-james.clark@arm.com>

Hi James

On Fri, 14 Jan 2022 at 17:33, James Clark <james.clark@arm.com> 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 doesn't go any further to validate if the user supplied value
> is valid, because none of the other parts this function do, but it does fix
> the case where it was impossible to ever set a valid value.
>

I concur that the input is not checked as valid, However all the other
fields are single bit - with no invalid values - other than the cond
field, which controls tracing of non-branch conditionals -
architecturally unsupported for A profile in ETMv4/.

> Signed-off-by: James Clark <james.clark@arm.com>
> ---
>  drivers/hwtracing/coresight/coresight-etm4x-sysfs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-sysfs.c b/drivers/hwtracing/coresight/coresight-etm4x-sysfs.c
> index a0640fa5c55b..a99bb537ea23 100644
> --- a/drivers/hwtracing/coresight/coresight-etm4x-sysfs.c
> +++ b/drivers/hwtracing/coresight/coresight-etm4x-sysfs.c
> @@ -368,7 +368,7 @@ static ssize_t mode_store(struct device *dev,
>         /* 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 ((mode & BIT(0)) && drvdata->q_support)
>                 config->cfg |= BIT(13);

This can be trivially changed to

if ((mode)  && drvdata->q_support)
         config->cfg |= BIT(13);

to ensure that any input mode 2'b01, 2'b10, 2'b11 results in output
settings of 2'b01, 2'b11, 2'b11 respectively

With that

Reviewed-by: Mike Leach <mike.leach@linaro.org>

>         /*
>          * if supported, Q elements with and without instruction
> --
> 2.17.1
>

-- 
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK