Re: [PATCH V1] PCI: designware-ep: Fix DBI access before core init

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

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.

Thanks,
Mani

> Thanks,
> Mani
> 
> > Rob
> 
> -- 
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