Date:   Tue, 4 Oct 2016 10:26:44 -0600
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Muhammad Abdul WAHAB <muhammadabdul.wahab@...elec.fr>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Adding missing features of Coresight PTM components

On 29 September 2016 at 04:16, Muhammad Abdul WAHAB
<muhammadabdul.wahab@...elec.fr> wrote:
> In the current driver for Coresight components, two features of PTM
> components are missing:
>
> 1. Branch Broadcasting (present also in ETM but called Branch Output)
> 2. Return Stack (only present in PTM v1.0 and PTMv1.1)
>
> These features can be added simply to the code using `mode` field of
> `etm_config` struct.

The rule of thumb when writing a patch description is to specify "why"
rather than "what" you are doing.  The above describe the why alright
and should be kept.  Everything below until your SOB describe what you
are doing and should be removed.

>
> ## Modifications in coresight-etm.h
> Two defines are added in register definition part of `coresight_etm.h` file
> that corresponds to the bitfield of these options. Two defines for mode
> field are added as well in the same file. The `ETM_MODE_ALL` field is
> modified accordingly.
>
> ## Modifs in coresight-etm3x-sysfs.c
> As the return stack feature is only available in PTM components, a test is
> made to make sure that for ETM components, this part is never executed.
> In addition, these two options (Branch Broadcasting and Return Stack) must
> not be enabled at the same time because the obtained trace is unpredictable
>  in this case (as described in
> [PFT architecture v1.1](https://goo.gl/lZ72R1)). For now, a warning is
> shown to alert user that the behavior is unpredictable. However, only one
> option could be allowed to trace. The user need to change configuration
> of PTM.
>
> To enable these features, the correct value should be written in `mode`
> file. The values are :
>
> 1. Branch Broadcasting (1 << 5)
> 2. Return Stack (1 << 6)
>
> Signed-off-by: Muhammad Abdul Wahab <muhammadabdul.wahab@...tralesupelec.fr>
> ---
> ### Purpose
>
> 1. **Branch Broadcast** : The branch broadcast feature is present in ETM
> components as well and is called Branch output. It allows to retrieve
> addresses for direct branch addresses alongside the indirect branch
> addresses. For example, it could be useful in cases when tracing without
> source code.
> 2. **Return Stack** : The return stack option allows to retrieve the return
>  addresses of function calls. It can be useful to avoid CRA
> (Code Reuse Attacks) by keeping a shadowstack.

The above should go in the patch description.

>
> ### Testing
>
> The trace sink components need to be enabled by accessing through sys file
>  system.
>
>     echo 1 > /sys/bus/coresight/devices/@...r.etb/enable\_sink
>
> Then enable the CS source component:
>
>     echo 1 > /sys/bus/coresight/devices/@...r.ptm/enable\_source
>
> By default, CS Source components are configured to trace the kernel.
>
> Then the trace can be read by dumping ETB.
>
>     dd if=/dev/@...r.etb of=trace_kernel.bin
>
> The trace can be visualized by
>
>     hexdump -C trace_kernel.bin
>
> Or stored using
>
>     hexdump -C trace_kernel.bin > trace_kernel.txt
>
> The trace need to be decoded to be readable.All these above steps can now
> be performed with Perf Library which was not available at the time I was
> playing with DT entries.

All of the above is not needed.

>
> diff -uprN -X linux-4.7-vanilla/Documentation/dontdiff
> linux-4.7-vanilla/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c
> linux-4.7/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c
> --- linux-4.7-vanilla/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c
> 2016-07-24 21:23:50.000000000 +0200
> +++ linux-4.7/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c
> 2016-09-28 15:36:39.886542702 +0200
> @@ -145,7 +145,7 @@ static ssize_t mode_store(struct device
>              goto err_unlock;
>          }
>          config->ctrl |= ETMCR_STALL_MODE;
> -     } else
> +    } else

There is indeed an indentation problem here but it can't be fixed in
this patch.  Please do another patch for this.

>          config->ctrl &= ~ETMCR_STALL_MODE;
>
>      if (config->mode & ETM_MODE_TIMESTAMP) {
> @@ -163,6 +163,20 @@ static ssize_t mode_store(struct device
>      else
>          config->ctrl &= ~ETMCR_CTXID_SIZE;
>
> +    if (config->mode & ETM_MODE_BBROAD)
> +        config->ctrl |= ETMCR_BRANCH_BROADCAST;
> +    else
> +        config->ctrl &= ~ETMCR_BRANCH_BROADCAST;
> +
> +    if (drvdata->arch == (PFT_ARCH_V1_0 | PFT_ARCH_V1_1)) {

The sysFS mode users can do what they want, including configurations
not supported by the HW.  There are so many rules and exception that
adding a check for every one of they doesn't scale up.  I suggest to
remove the above check.

> +        if (config->mode & ETM_MODE_RET_STACK) {
> +            if (config->mode & ETM_MODE_BBROAD)
> +                dev_warn(drvdata->dev, "behavior is unpredictable\n");
> +            config->ctrl |= ETMCR_RETURN_STACK_EN;
> +        } else
> +            config->ctrl &= ~ETMCR_RETURN_STACK_EN;
> +    }
> +
>      if (config->mode & (ETM_MODE_EXCL_KERN | ETM_MODE_EXCL_USER))
>          etm_config_trace_mode(config);
>
> diff -uprN -X linux-4.7-vanilla/Documentation/dontdiff
> linux-4.7-vanilla/drivers/hwtracing/coresight/coresight-etm.h
> linux-4.7/drivers/hwtracing/coresight/coresight-etm.h
> --- linux-4.7-vanilla/drivers/hwtracing/coresight/coresight-etm.h
> 2016-07-24 21:23:50.000000000 +0200
> +++ linux-4.7/drivers/hwtracing/coresight/coresight-etm.h    2016-09-28
> 15:35:59.862544418 +0200
> @@ -89,11 +89,13 @@
>  /* ETMCR - 0x00 */
>  #define ETMCR_PWD_DWN        BIT(0)
>  #define ETMCR_STALL_MODE    BIT(7)
> +#define ETMCR_BRANCH_BROADCAST    BIT(8)
>  #define ETMCR_ETM_PRG        BIT(10)
>  #define ETMCR_ETM_EN        BIT(11)
>  #define ETMCR_CYC_ACC        BIT(12)
>  #define ETMCR_CTXID_SIZE    (BIT(14)|BIT(15))
>  #define ETMCR_TIMESTAMP_EN    BIT(28)
> +#define ETMCR_RETURN_STACK_EN    BIT(29)  /* PTM v1.0 & PTM v1.1 */
>  /* ETMCCR - 0x04 */
>  #define ETMCCR_FIFOFULL        BIT(23)
>  /* ETMPDCR - 0x310 */
> @@ -110,8 +112,11 @@
>  #define ETM_MODE_STALL        BIT(2)
>  #define ETM_MODE_TIMESTAMP    BIT(3)
>  #define ETM_MODE_CTXID        BIT(4)
> +#define ETM_MODE_BBROAD    BIT(5)
> +#define ETM_MODE_RET_STACK    BIT(6)
>  #define ETM_MODE_ALL        (ETM_MODE_EXCLUDE | ETM_MODE_CYCACC | \
>                   ETM_MODE_STALL | ETM_MODE_TIMESTAMP | \
> +                 ETM_MODE_BBROAD | ETM_MODE_RET_STACK | \
>                   ETM_MODE_CTXID | ETM_MODE_EXCL_KERN | \
>                   ETM_MODE_EXCL_USER)

You patch doesn't apply on my tree.  Please use "git format-patch" for
your next submission.  Last but not least the email address in the
"from:" part of the submission doesn't match the one in the SOB - they
have to be similar.

Thanks,
Mathieu

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