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

[Sai Prakash Ranjan]

Hi Mathieu,

On 2020-04-15 21:26, Mathieu Poirier wrote:
> On Tue, 14 Apr 2020 at 09:47, Sai Prakash Ranjan
> <saiprakash.ranjan@xxxxxxxxxxxxxx> 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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