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Wed, 3 Jun 2020 13:43:10 +0000 (UTC) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Wed, 03 Jun 2020 19:13:10 +0530 From: Sai Prakash Ranjan To: Robin Murphy , Mike Leach Cc: Mathieu Poirier , Suzuki K Poulose , linux-arm-kernel , Linux Kernel Mailing List , linux-arm-msm@vger.kernel.org, Coresight ML , Stephen Boyd Subject: Re: [PATCH 2/2] coresight: tmc: Add shutdown callback for TMC ETR/ETF In-Reply-To: <1a5a6a6d-b86d-df45-cf91-7081e70d88a3@arm.com> References: <28123d1e19f235f97555ee36a5ed8b52d20cbdea.1590947174.git.saiprakash.ranjan@codeaurora.org> <20200601212858.GB24287@xps15> <6d759cc28628ea72767c1304883630eb@codeaurora.org> <4a09cd2e054836d85f2e024ca4435e91@codeaurora.org> <1a5a6a6d-b86d-df45-cf91-7081e70d88a3@arm.com> Message-ID: <07851ea3d1abfe60bb6c1761bf19f62f@codeaurora.org> X-Sender: saiprakash.ranjan@codeaurora.org User-Agent: Roundcube Webmail/1.3.9 Sender: linux-arm-msm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-arm-msm@vger.kernel.org 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 >> wrote: >>> >>> Hi Mike, >>> >>> On 2020-06-03 16:57, Mike Leach wrote: >>>> Hi, >>>> >>>> On Wed, 3 Jun 2020 at 11:24, Sai Prakash Ranjan >>>> 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 -- QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation