Messages in this thread | | | Date | Wed, 03 Jun 2020 19:13:10 +0530 | From | Sai Prakash Ranjan <> | Subject | Re: [PATCH 2/2] coresight: tmc: Add shutdown callback for TMC ETR/ETF |
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Hi Mike,
On 2020-06-03 19:04, 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. >
As Robin said, it is not SMMU callback but the normal reboot/shutdown flow and race is unlikely at that point.
tmc_shutdown() platform_drv_shutdown() device_shutdown() kernel_restart_prepare() kernel_restart()
If I am not clear enough, first all the consumer shutdown callbacks of SMMU are called like above tmc_shutdown() and then we call the arm_smmu_device_shutdown(), this ordering is ensured by the device links.
Thanks, Sai
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