Messages in this thread | | | Date | Thu, 04 Jun 2020 12:57:56 +0530 | From | Sai Prakash Ranjan <> | Subject | Re: [PATCH 2/2] coresight: tmc: Add shutdown callback for TMC ETR/ETF |
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Hi Mathieu,
+Will
On 2020-06-03 23:14, Mathieu Poirier wrote: > On Wed, Jun 03, 2020 at 02:34:10PM +0100, Robin Murphy wrote: >> On 2020-06-03 14:22, Mike Leach wrote: >> > Hi Sai, >> > >> > On Wed, 3 Jun 2020 at 13:14, Sai Prakash Ranjan >> > <saiprakash.ranjan@codeaurora.org> wrote: >> > > >> > > Hi Mike, >> > > >> > > On 2020-06-03 16:57, Mike Leach wrote: >> > > > Hi, >> > > > >> > > > On Wed, 3 Jun 2020 at 11:24, Sai Prakash Ranjan >> > > > <saiprakash.ranjan@codeaurora.org> wrote: >> > > > > >> > > > > Hi Mike, >> > > > > >> > > > > Thanks again for looking at this. >> > > > > >> > > > > On 2020-06-03 03:42, Mike Leach wrote: >> > > > > [...] >> > > > > >> > > > > > > >> > > > > > > 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. >> > > > > > >> > > > > > I think this use case can be handled slightly differently than the >> > > > > > general requirements for modular CoreSight drivers. >> > > > > > >> > > > > > What is needed here is a way of stopping the underlying ETR hardware >> > > > > > from issuing data to the SMMU, until the entire device has been shut >> > > > > > down, in a way that does not remove the driver, breaking existing >> > > > > > references and causing a system crash. >> > > > > > >> > > > > > We could introduce a new mode to the ETR driver - e.g. >> > > > > > CS_MODE_SHUTDOWN. >> > > > > > >> > > > > > At the end of the block tmc_shutdown(struct amba_device *adev), set >> > > > > > drvdata->mode to CS_MODE_SHUTDOWN & remove the coresight_unregister(). >> > > > > > This new mode can be used to prevent the underlying hardware from >> > > > > > being able to restart until the device is re-powered. >> > > > > > >> > > > > > This mode can be detected in the code that enables / disables the ETR >> > > > > > and handled appropriately (updates to tmc_enable_etr_sink and >> > > > > > tmc_disable_etr_sink). >> > > > > > This mode will persist until the device is re-started - but because we >> > > > > > are on the device shutdown path this is not an issue. >> > > > > > >> > > > > > This should leave the CoreSight infrastructure stable until the >> > > > > > drivers are shut down normally as part of the device power down >> > > > > > process. >> > > > > > >> > > > > >> > > > > Sounds good to me, but if the coresight_unregister() is the trouble >> > > > > point >> > > > > causing these crashes, then can't we just remove that from >> > > > > tmc_shutdown() >> > > > > callback? This would be like maintaining the same behaviour as now >> > > > > where >> > > > > on reboot/shutdown we basically don't do anything except for disabling >> > > > > ETR. >> > > > >> > > > No - the new mode prevents race conditions where the thread shutting >> > > > down the SMMU does the ETR shutdown, but then another thread happens >> > > > to be trying to start trace and restarts the ETR. >> > > > It also prevents the condition Mathieu discussed where a thread might >> > > > be attempting to shutdown trace - this could try to disable the >> > > > hardware again re-releasing resources/ re-flushing and waiting for >> > > > stop. >> > > > >> > > >> > > I do not think there will a race between SMMU shutdown and ETR shutdown. >> > > Driver core takes care of calling SMMU shutdown after its consumer >> > > shutdown callbacks via device link, otherwise there would already be >> > > bugs in all other client drivers. >> > > >> > >> > I am not saying there could be a race between tmc_shutdowm and >> > Smmu_shutdown - there may be a case if the coresight_disable_path >> > sequence is running and gets to the point of disabling the ETR after >> > the SMMU callback has disabled it. >> >> I'm confused now - there is no "SMMU callback", we're talking about >> the >> system-wide cleanup from kernel_shutdown_prepare() or >> kernel_restart_prepare(). As far as I'm aware userspace should be long >> gone >> by that point, so although trace may have been left running, the >> chance of >> racing against other driver operations seems pretty unlikely. > > Robin has a point - user space is long gone at this time. As such the > first > question to ask is what kind of CS session was running at the time the > system > was shutting down. Was it a perf session of a sysfs session? > > I'm guessing it was a sysfs session because user space has been blown > away a > while back and part of that process should have killed all perf > sessions.
I was enabling trace via sysfs.
> > If I am correct then simply switching off the ETR HW in the shutdown() > amba bus > callback should be fine - otherwise Mike's approach is mandatory. > There is > also the exchange between Robin and Sai about removing the SMMU > shutdown > callback, but that thread is still incomplete. >
If Robin is hinting at removing SMMU shutdown callback, then I think adding all these shutdown callbacks to all clients of SMMU can be avoided. Git blaming the thing shows it was added to avoid some kexec memory corruption.
Thanks, Sai
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