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

From: Sai Prakash Ranjan <[email-masked]>
Date: Tue, 14 Apr 2020 21:17:05 +0530

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;
+       }
+
         /* There is no point in reading a TMC in HW FIFO mode */
         mode = readl_relaxed(drvdata->base + TMC_MODE);
         if (mode != TMC_MODE_CIRCULAR_BUFFER) {

Thanks,
Sai

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