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Wed, 3 Jun 2020 14:02:01 +0000 (UTC) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Wed, 03 Jun 2020 19:32:01 +0530 From: Sai Prakash Ranjan To: Mike Leach Cc: Robin Murphy , Mathieu Poirier , Suzuki K Poulose , linux-arm-msm@vger.kernel.org, Coresight ML , Linux Kernel Mailing List , Stephen Boyd , linux-arm-kernel Subject: Re: [PATCH 2/2] coresight: tmc: Add shutdown callback for TMC ETR/ETF In-Reply-To: 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: 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 19:21, Mike Leach wrote: > Hi, > > > On Wed, 3 Jun 2020 at 14:34, 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 || > ((offset >= TRCCIDCVRn(0)) && (offset <= TRCVMIDCVRn(7)), > the >> chance of racing against other driver operations seems pretty >> unlikely. >> > > Sorry - bad choice of terminology. I was referring to the SMMU > ensuring that it had all its clients shut-down before if shut down. To > quote Sai... > >>>>>> SMMU device link just makes sure that >> >>>>>> SMMU(supplier) shutdown is >> >>>>>> called only after its consumers shutdown callbacks are called. > > I agree it is unlikely, but if removing the device from the CoreSight > infrastructure via coresight_unregister() is a potential source of a > crash, it would seem that there is a potential path where some > CoreSight driver side work might be possible. therefore a mode to > prevent this crash, and ensure that the device hardware remains off > and not sending trace to SMMU until such time as shutdown / reboot > restart occurs, seemed prudent. > Actually I did not see any crash with coresight_unregister() during reboot/shutdown as I mentioned previously to Mathieu's query on this being similar to remove callback. I think the crash with coresight_unregister() is only seen when we have coresight as module and the userspace is pretty much there to enable/disable trace when we try to bind/unbind. But here we only consider the system reboot/shutdown where pretty much everything is down by this point. Thanks, Sai -- QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation