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Date:   Fri, 19 Aug 2022 14:05:41 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
To:     Vidya Sagar <vidyas@nvidia.com>
Cc:     Lorenzo Pieralisi <lpieralisi@kernel.org>,
        Rob Herring <robh@kernel.org>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Jingoo Han <jingoohan1@gmail.com>,
        Gustavo Pimentel <gustavo.pimentel@synopsys.com>,
        Krzysztof Wilczynski <kw@linux.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        PCI <linux-pci@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Krishna Thota <kthota@nvidia.com>,
        Manikanta Maddireddy <mmaddireddy@nvidia.com>,
        sagar.tv@gmail.com, Xiaowei Bao <xiaowei.bao@nxp.com>,
        Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Subject: Re: [PATCH V1] PCI: designware-ep: Fix DBI access before core init
Message-ID: <20220819083541.GB215264@thinkpad>
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On Tue, Aug 16, 2022 at 08:05:08PM +0530, Vidya Sagar wrote:
> 
> 
> On 8/16/2022 7:45 PM, Lorenzo Pieralisi wrote:
> > External email: Use caution opening links or attachments
> > 
> > 
> > On Tue, Aug 02, 2022 at 07:37:38PM +0530, Manivannan Sadhasivam wrote:
> > > On Tue, Aug 02, 2022 at 12:54:37PM +0530, Manivannan Sadhasivam wrote:
> > > > On Mon, Aug 01, 2022 at 02:27:14PM -0600, Rob Herring wrote:
> > > > > On Sat, Jul 30, 2022 at 8:50 AM Manivannan Sadhasivam
> > > > > <manivannan.sadhasivam@linaro.org> wrote:
> > > > > > 
> > > > > > On Fri, Jul 29, 2022 at 05:44:04PM -0500, Bjorn Helgaas wrote:
> > > > > > > [+cc Xiaowei (author of 6bfc9c3a2c70), Hou (author of 8bcca2658558)]
> > > > > > > 
> > > > > > > On Thu, Jul 28, 2022 at 05:56:28PM +0530, Vidya Sagar wrote:
> > > > > > > > On 7/28/2022 3:44 AM, Bjorn Helgaas wrote:
> > > > > > > > > On Wed, Jun 22, 2022 at 09:31:33AM +0530, Vidya Sagar wrote:
> > > > > > > > > > Platforms that cannot support their core initialization without the
> > > > > > > > > > reference clock from the host, implement the feature 'core_init_notifier'
> > > > > > > > > > to indicate the DesignWare sub-system about when their core is getting
> > > > > > > > > > initialized. Any accesses to the core (Ex:- DBI) would result in system
> > > > > > > > > > hang in such systems (Ex:- tegra194). This patch moves any access to the
> > > > > > > > > > core to dw_pcie_ep_init_complete() API which is effectively called only
> > > > > > > > > > after the core initialization.
> > > > > 
> > > > > > >    6) What's going on with the CORE_INIT and LINK_UP notifiers?
> > > > > > >       dw_pcie_ep_init_notify() is only called by qcom and tegra.
> > > > > > >       dw_pcie_ep_linkup() is only called by dra7xx, qcom, and tegra.
> > > > > > >       As far as I can tell, nobody at all registers to handle those
> > > > > > >       events except a test.  I think it's pointless to have that code
> > > > > > >       if nobody uses it.
> > > > > > > 
> > > > > > 
> > > > > > I have submitted an actual driver that makes use of these notifiers:
> > > > > > https://lore.kernel.org/lkml/20220502060611.58987-9-manivannan.sadhasivam@linaro.org/
> > > > > 
> > > > > Notifiers aren't the best interface in the kernel. I think they are
> > > > > best used if there's no real linkage between the sender and receiver.
> > > > > For an EPC and EPF that's a fixed interface, so define a proper
> > > > > interface.
> > > > > 
> > > > 
> > > > Fair point! The use of notifiers also suffer from an issue where the notifier
> > > > chain in EPC is atomic but the EPF calls some of the functions like
> > > > pci_epc_write_header() could potentially sleep.
> > > > 
> > > > I'll try to come up with an interface.
> > > > 
> > > 
> > > I thought about using a new set of callbacks that define the EPC events and
> > > have the EPF drivers populate them during probe time. Like below,
> > > 
> > > ```
> > > diff --git a/include/linux/pci-epf.h b/include/linux/pci-epf.h
> > > index e03c57129ed5..45247802d6f7 100644
> > > --- a/include/linux/pci-epf.h
> > > +++ b/include/linux/pci-epf.h
> > > @@ -74,6 +74,20 @@ struct pci_epf_ops {
> > >                                          struct config_group *group);
> > >   };
> > > 
> > > +/**
> > > + * struct pci_epf_events - Callbacks for capturing the EPC events
> > > + * @init_complete: Callback for the EPC initialization complete event
> > > + * @link_up: Callback for the EPC link up event
> > > + */
> > > +struct pci_epc_events {
> > > +       void (*init_complete)(struct pci_epf *epf);
> > > +       void (*link_up)(struct pci_epf *epf);
> > > +};
> > > +
> > >   /**
> > >    * struct pci_epf_driver - represents the PCI EPF driver
> > >    * @probe: ops to perform when a new EPF device has been bound to the EPF driver
> > > @@ -172,6 +186,7 @@ struct pci_epf {
> > >          unsigned int            is_vf;
> > >          unsigned long           vfunction_num_map;
> > >          struct list_head        pci_vepf;
> > > +       struct pci_epc_events   *events;
> > >   };
> > > 
> > >   /**
> > > ```
> > > 
> > > When each of the event is received by the EPC driver, it will use the EPC API
> > > to call the relevant event callback for _each_ EPF. Like below:
> > > 
> > > ```
> > > diff --git a/drivers/pci/endpoint/pci-epc-core.c b/drivers/pci/endpoint/pci-epc-core.c
> > > index 6ad9b38b63a9..4b0b30b91403 100644
> > > --- a/drivers/pci/endpoint/pci-epc-core.c
> > > +++ b/drivers/pci/endpoint/pci-epc-core.c
> > > @@ -724,10 +724,15 @@ EXPORT_SYMBOL_GPL(pci_epc_linkdown);
> > >    */
> > >   void pci_epc_init_notify(struct pci_epc *epc)
> > >   {
> > > +       struct pci_epf *epf;
> > > +
> > >          if (!epc || IS_ERR(epc))
> > >                  return;
> > > 
> > > -       blocking_notifier_call_chain(&epc->notifier, CORE_INIT, NULL);
> > > +       list_for_each_entry(epf, &epc->pci_epf, list) {
> > > +               if (epf->events->init_complete)
> > > +                       epf->events->init_complete(epf);
> > > +       }
> > >   }
> > >   EXPORT_SYMBOL_GPL(pci_epc_init_notify);
> > > ```
> > > 
> > > Does this look good to you? I can spin up an RFC series, but wanted to check the
> > > interface design beforehand.
> > 
> > I am resuming patch reviews, have you posted a follow up ?
> > 
> > Just to understand where we are with this thread and start reviewing
> > from there, I will update patchwork accordingly (you should add
> > a Link: to this thread anyway in the new series).
> 
> Manivannan posted a new patch set at
> https://patchwork.kernel.org/project/linux-pci/list/?series=666818 to
> address concerns with the notifier mechanism.
> 
> I would be sending a follow-up patch for the current patch soon.
> 

While posting the new revision, please move dw_pcie_edma_detect() and ep_init()
to init_complete function. They also perform DBI access.

Thanks,
Mani

> Thanks,
> Vidya Sagar
> 
> > 
> > Thanks,
> > Lorenzo
> > 
> > > Thanks,
> > > Mani
> > > 
> > > > Thanks,
> > > > Mani
> > > > 
> > > > > Rob
> > > > 
> > > > --
> > > > மணிவண்ணன் சதாசிவம்
> > > 
> > > --
> > > மணிவண்ணன் சதாசிவம்

-- 
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