Hi Sai, On Wed, 3 Jun 2020 at 13:14, Sai Prakash Ranjan <saiprakash.ranjan@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx> 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. Mike > Thanks, > Sai > -- > QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a > member > of Code Aurora Forum, hosted by The Linux Foundation -- Mike Leach Principal Engineer, ARM Ltd. Manchester Design Centre. UK
