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[216.228.112.22]) by smtp.gmail.com with ESMTPSA id b1sm10805631pft.127.2020.10.09.09.01.19 (version=TLS1_2 cipher=ECDHE-ECDSA-CHACHA20-POLY1305 bits=256/256); Fri, 09 Oct 2020 09:01:19 -0700 (PDT) Date: Fri, 9 Oct 2020 08:54:09 -0700 From: Nicolin Chen To: Thierry Reding Subject: Re: [PATCH v4 2/3] iommu/tegra-smmu: Rework tegra_smmu_probe_device() Message-ID: <20201009155409.GB4469@Asurada-Nvidia> References: <20201002060807.32138-1-nicoleotsuka@gmail.com> <20201002060807.32138-3-nicoleotsuka@gmail.com> <5542b314-f414-1e83-8cf6-2bf22a41ae9c@gmail.com> <20201002185828.GC29706@Asurada-Nvidia> <20201005095754.GJ425362@ulmo> <20201006010546.GB28640@Asurada-Nvidia> <20201008095343.GA2349275@ulmo> <20201008211209.GC32140@Asurada-Nvidia> <20201009122556.GE458338@ulmo> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20201009122556.GE458338@ulmo> User-Agent: Mutt/1.9.4 (2018-02-28) Cc: linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org, jonathanh@nvidia.com, 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[216.228.112.22]) by smtp.gmail.com with ESMTPSA id b1sm10805631pft.127.2020.10.09.09.01.19 (version=TLS1_2 cipher=ECDHE-ECDSA-CHACHA20-POLY1305 bits=256/256); Fri, 09 Oct 2020 09:01:19 -0700 (PDT) Date: Fri, 9 Oct 2020 08:54:09 -0700 From: Nicolin Chen To: Thierry Reding Cc: Dmitry Osipenko , joro@8bytes.org, vdumpa@nvidia.com, jonathanh@nvidia.com, linux-tegra@vger.kernel.org, iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 2/3] iommu/tegra-smmu: Rework tegra_smmu_probe_device() Message-ID: <20201009155409.GB4469@Asurada-Nvidia> References: <20201002060807.32138-1-nicoleotsuka@gmail.com> <20201002060807.32138-3-nicoleotsuka@gmail.com> <5542b314-f414-1e83-8cf6-2bf22a41ae9c@gmail.com> <20201002185828.GC29706@Asurada-Nvidia> <20201005095754.GJ425362@ulmo> <20201006010546.GB28640@Asurada-Nvidia> <20201008095343.GA2349275@ulmo> <20201008211209.GC32140@Asurada-Nvidia> <20201009122556.GE458338@ulmo> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20201009122556.GE458338@ulmo> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-tegra@vger.kernel.org On Fri, Oct 09, 2020 at 02:25:56PM +0200, Thierry Reding wrote: > On Thu, Oct 08, 2020 at 02:12:10PM -0700, Nicolin Chen wrote: > > On Thu, Oct 08, 2020 at 11:53:43AM +0200, Thierry Reding wrote: > > > On Mon, Oct 05, 2020 at 06:05:46PM -0700, Nicolin Chen wrote: > > > > On Mon, Oct 05, 2020 at 11:57:54AM +0200, Thierry Reding wrote: > > > > > On Fri, Oct 02, 2020 at 11:58:29AM -0700, Nicolin Chen wrote: > > > > > > On Fri, Oct 02, 2020 at 06:02:18PM +0300, Dmitry Osipenko wrote: > > > > > > > 02.10.2020 09:08, Nicolin Chen пишет: > > > > > > > > static int tegra_smmu_of_xlate(struct device *dev, > > > > > > > > struct of_phandle_args *args) > > > > > > > > { > > > > > > > > + struct platform_device *iommu_pdev = of_find_device_by_node(args->np); > > > > > > > > + struct tegra_mc *mc = platform_get_drvdata(iommu_pdev); > > > > > > > > u32 id = args->args[0]; > > > > > > > > > > > > > > > > + of_node_put(args->np); > > > > > > > > > > > > > > of_find_device_by_node() takes device reference and not the np > > > > > > > reference. This is a bug, please remove of_node_put(). > > > > > > > > > > > > Looks like so. Replacing it with put_device(&iommu_pdev->dev); > > > > > > > > > > Putting the put_device() here is wrong, though. You need to make sure > > > > > you keep a reference to it as long as you keep accessing the data that > > > > > is owned by it. > > > > > > > > I am confused. You said in the other reply (to Dmitry) that we do > > > > need to put_device(mc->dev), where mc->dev should be the same as > > > > iommu_pdev->dev. But here your comments sounds that we should not > > > > put_device at all since ->probe_device/group_device/attach_dev() > > > > will use it later. > > > > > > You need to call put_device() at some point to release the reference > > > that you acquired by calling of_find_device_by_node(). If you don't > > > release it, you're leaking the reference and the kernel isn't going to > > > know when it's safe to delete the device. > > > > > > So what I'm saying is that we either release it here, which isn't quite > > > right because we do reference data relating to the device later on. And > > > > I see. A small question here by the way: By looking at other IOMMU > > drivers that are calling driver_find_device_by_fwnode() function, > > I found that most of them put_device right after the function call, > > and dev_get_drvdata() after putting the device.. > > > > Feels like they are doing it wrongly? > > Well, like I said this is somewhat academic because these are all > referencing the IOMMU that by definition still needs to be around > when this code is called, and there's locks in place to ensure > these don't go away. So it's not like these drivers are doing it > wrong, they're just not doing it pedantically right. > > > > > > because it isn't quite right there should be a reason to justify it, > > > which is that the SMMU parent device is the same as the MC, so the > > > reference count isn't strictly necessary. But that's not quite obvious, > > > so highlighting it in a comment makes sense. > > > > > > The other alternative is to not call put_device() here and keep on to > > > the reference as long as you keep using "mc". This might be difficult to > > > implement because it may not be obvious where to release it. I think > > > this is the better alternative, but if it's too complicated to implement > > > it might not be worth it. > > > > I feel so too. The dev is got at of_xlate() that does not have an > > obvious counterpart function. So I'll just remove put_device() and > > put a line of comments, as you suggested. > > I think you misunderstood. Not calling put_device() would be wrong > because that leaks a reference to the SMMU that you can't get back. My > suggestion was rather to keep put_device() here, but add a comment as to > why it's okay to call the put_device() here, even though you keep using > its private data later beyond this point, which typically would be wrong > to do. I see. Thanks for clarification! Will send v6 soon.