Subject: Re: [PATCH] coresight: tmc: Read TMC mode only when TMC hw is enabled

From: Sai Prakash Ranjan <[email-masked]>
Date: Thu, 16 Apr 2020 21:47:34 +0530

Hi Mathieu,

On 2020-04-15 21:26, Mathieu Poirier wrote:
> On Tue, 14 Apr 2020 at 09:47, Sai Prakash Ranjan
> <[email-masked]> wrote:
>> 
>> Hi Mathieu,
>> 
>> On 2020-04-13 22:44, Mathieu Poirier wrote:
>> > On Mon, Apr 13, 2020 at 01:55:30PM +0530, Sai Prakash Ranjan wrote:
>> >> Hi Suzuki,
>> >>
>> >> On 2020-04-13 04:47, Suzuki K Poulose wrote:
>> >> > Hi Sai,
>> >> >
>> >> > On 04/09/2020 12:35 PM, Sai Prakash Ranjan wrote:
>> >> > > Reading TMC mode register in tmc_read_prepare_etb without
>> >> > > enabling the TMC hardware leads to async exceptions like
>> >> > > the one in the call trace below. This can happen if the
>> >> > > user tries to read the TMC etf data via device node without
>> >> > > setting up source and the sink first which enables the TMC
>> >> > > hardware in the path. So make sure that the TMC is enabled
>> >> > > before we try to read TMC data.
>> >> >
>> >> > So, one can trigger the same SError by simply :
>> >> >
>> >> > $ cat /sys/bus/coresight/device/tmc_etb0/mgmt/mode
>> >> >
>> >>
>> >> I do not see any SError when I run the above command.
>> >>
>> >> localhost ~ # cat /sys/bus/coresight/devices/tmc_etf0/mgmt/mode
>> >> 0x0
>> >>
>> >> And this is most likely due to
>> >>
>> >> commit cd9e3474bb793dc ("coresight: add PM runtime calls to
>> >> coresight_simple_func()")
>> >
>> > Ok, so this is related to power management (you can ignore my question
>> > in the
>> > previous email).
>> >
>> > Regarding function tmc_read_prepare_etb(), the best way to deal with
>> > this is
>> > probably make sure drvdata->mode != CS_MODE_DISABLED before reading
>> > TMC_MODE.
>> > If there is a buffer to read it will have been copied when the ETB was
>> > disabled
>> > and there won't be a need to access the HW.
>> >
>> 
>> This works as well, thanks.
>> 
>> diff --git a/drivers/hwtracing/coresight/coresight-tmc-etf.c
>> b/drivers/hwtracing/coresight/coresight-tmc-etf.c
>> index d0cc3985b72a..7ffe05930984 100644
>> --- a/drivers/hwtracing/coresight/coresight-tmc-etf.c
>> +++ b/drivers/hwtracing/coresight/coresight-tmc-etf.c
>> @@ -596,6 +596,11 @@ int tmc_read_prepare_etb(struct tmc_drvdata
>> *drvdata)
>>                  goto out;
>>          }
>> 
>> +       if (drvdata->mode == CS_MODE_DISABLED) {
>> +               ret = -EINVAL;
>> +               goto out;
>> +       }
>> +
> 
> We are back to your original solution where the ETB buffer can't be
> read if the ETB itself is not enabled.  It _is_ possible to read the
> buffer of an ETB that has been disabled.
> 
> To fix this consider the following [1].  Take the block at line 607
> and move it to line 598.  As part of the if() condition at line 619,
> read the value of the TMC_MODE register and exit if not in circular
> mode.  If it is in circular mode continue with disabling the hardware.
> 
> [1].
> https://elixir.bootlin.com/linux/v5.7-rc1/source/drivers/hwtracing/coresight/coresight-tmc-etf.c
> 

Thanks, got it now. Posted v2 - 
https://lore.kernel.org/patchwork/patch/1226022/

-Sai

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