Re: [PATCH 2/2] coresight: tmc: Add shutdown callback for TMC ETR/ETF

From: Sai Prakash Ranjan
Date: Tue Jun 02 2020 - 03:30:32 EST

Hi Mathieu,

Thanks for taking your time for review.

On 2020-06-02 02:58, Mathieu Poirier wrote:
Hi Sai,

On top of the comments already privided by Mike, I have the following:

On Mon, Jun 01, 2020 at 01:32:26PM +0530, Sai Prakash Ranjan wrote:
Implement a shutdown callback to ensure ETR/ETF hardware is
properly shutdown in reboot/shutdown path. This is required
for ETR/ETF which has SMMU address translation enabled like
on SC7180 SoC and few others. If the hardware is still accessing
memory after SMMU translation is disabled as part of SMMU
shutdown callback in system reboot or shutdown path, then
IOVAs(I/O virtual address) which it was using will go on the bus
as the physical addresses which might result in unknown crashes
(NoC/interconnect errors). So we make sure from this shutdown
callback that the ETR/ETF is shutdown before SMMU translation is
disabled and device_link in SMMU driver will take care of ordering
of shutdown callbacks such that SMMU shutdown callback is not
called before any of its consumer shutdown callbacks.

Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@xxxxxxxxxxxxxx>
.../hwtracing/coresight/coresight-tmc-etf.c | 4 +--
.../hwtracing/coresight/coresight-tmc-etr.c | 2 +-
drivers/hwtracing/coresight/coresight-tmc.c | 29 +++++++++++++++++++
drivers/hwtracing/coresight/coresight-tmc.h | 3 ++
4 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-tmc-etf.c b/drivers/hwtracing/coresight/coresight-tmc-etf.c
index 36cce2bfb744..cba3e7592820 100644
--- a/drivers/hwtracing/coresight/coresight-tmc-etf.c
+++ b/drivers/hwtracing/coresight/coresight-tmc-etf.c
@@ -85,7 +85,7 @@ static void __tmc_etb_disable_hw(struct tmc_drvdata *drvdata)

-static void tmc_etb_disable_hw(struct tmc_drvdata *drvdata)
+void tmc_etb_disable_hw(struct tmc_drvdata *drvdata)
@@ -118,7 +118,7 @@ static int tmc_etf_enable_hw(struct tmc_drvdata *drvdata)
return 0;

-static void tmc_etf_disable_hw(struct tmc_drvdata *drvdata)
+void tmc_etf_disable_hw(struct tmc_drvdata *drvdata)

Why do we care about ETB and ETF when they both use RAM internal to the device?
Moreover, the system RAM they use is not dedicated and as such falls back to the
kernel's memory pool.

Actually we don't, I added the disable for ETF/ETB for completeness since we are
adding shutdown callback for TMC devices and not just ETR although this issue applies
only for ETR and it doesn't hurt to disable these devices in shutdown path.

diff --git a/drivers/hwtracing/coresight/coresight-tmc-etr.c b/drivers/hwtracing/coresight/coresight-tmc-etr.c
index 625882bc8b08..b29c2db94d96 100644
--- a/drivers/hwtracing/coresight/coresight-tmc-etr.c
+++ b/drivers/hwtracing/coresight/coresight-tmc-etr.c
@@ -1110,7 +1110,7 @@ static void __tmc_etr_disable_hw(struct tmc_drvdata *drvdata)


-static void tmc_etr_disable_hw(struct tmc_drvdata *drvdata)
+void tmc_etr_disable_hw(struct tmc_drvdata *drvdata)
/* Disable CATU device if this ETR is connected to one */
diff --git a/drivers/hwtracing/coresight/coresight-tmc.c b/drivers/hwtracing/coresight/coresight-tmc.c
index 5a271ebc4585..7e687a356fe0 100644
--- a/drivers/hwtracing/coresight/coresight-tmc.c
+++ b/drivers/hwtracing/coresight/coresight-tmc.c
@@ -540,6 +540,34 @@ static int tmc_probe(struct amba_device *adev, const struct amba_id *id)
return ret;

+static void tmc_shutdown(struct amba_device *adev)
+ struct tmc_drvdata *drvdata = amba_get_drvdata(adev);
+ if (!drvdata->enable)
+ goto out;
+ /*
+ * We do not care about the active trace sessions here
+ * since the system is going down unlike remove callback,
+ * just make sure that the hardware is shutdown.
+ */
+ switch (drvdata->config_type) {
+ tmc_etb_disable_hw(drvdata);
+ break;
+ tmc_etf_disable_hw(drvdata);
+ break;
+ tmc_etr_disable_hw(drvdata);
+ }
+ misc_deregister(&drvdata->miscdev);
+ coresight_unregister(drvdata->csdev);

If a session is active when tmc_shutdown() is called, unregistering the ETF/ETR
will result in a kernel crash if the session is stopped before the kernel has
had the opportunity to shutdown. It is the problem as trying to make coresight
drivers modular.

For this to really work the ongoing session would need to be stopped. That
would teardown the path and stop the sink.

I have tested this with and without active trace sessions multiple times on 2 devices
and did not observe a single crash. The crash should be easily triggered as per
what you are saying if we have active sessions but I do not see any crash.

That being said I'm sure that dependencies on an IOMMU isn't a problem confined
to coresight. I am adding Robin Murphy, who added this commit [1], to the thread
in the hope that he can provide guidance on the right way to do this.

SMMU/IOMMU won't be able to do much here as it is the client's responsiblity to
properly shutdown and SMMU device link just makes sure that SMMU(supplier) shutdown is
called only after its consumers shutdown callbacks are called.


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