Date:   Wed, 10 Aug 2022 12:16:18 -0600
From:   Rob Herring <robh@...nel.org>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Vidya Sagar <vidyas@...dia.com>,
        Jingoo Han <jingoohan1@...il.com>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczynski <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        PCI <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Krishna Thota <kthota@...dia.com>,
        Manikanta Maddireddy <mmaddireddy@...dia.com>,
        sagar.tv@...il.com, Xiaowei Bao <xiaowei.bao@....com>,
        Hou Zhiqiang <Zhiqiang.Hou@....com>
Subject: Re: [PATCH V1] PCI: designware-ep: Fix DBI access before core init

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@...aro.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 {

pci_epc_event_ops

> +       void (*init_complete)(struct pci_epf *epf);
> +       void (*link_up)(struct pci_epf *epf);

link down?

> +};
> +
>  /**
>   * 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;

*event_ops

And 'const'

>  };
>  
>  /**
> ```
> 
> 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 guess. Honestly, the I'm not all that familiar with the endpoint 
stuff.

Rob

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