Re: [v6 3/5] PCI: cadence: Use common PCI host bridge APIs for finding the capabilities

[Hans Zhang]

On 2025/3/25 20:16, Hans Zhang wrote:
> > > > > > I'm really wondering why the read config function is provided 
> > > > > > directly
> > > > > > as
> > > > > > an argument. Shouldn't struct pci_host_bridge have some ops that can
> > > > > > read
> > > > > > config so wouldn't it make much more sense to pass it and use the 
> > > > > > func
> > > > > > from there? There seems to ops in pci_host_bridge that has read(), 
> > > > > > does
> > > > > > that work? If not, why?
> > > > >
> > > > > No effect.
> > > >
> > > > I'm not sure what you meant?
> > > >
> > > > > Because we need to get the offset of the capability before PCIe
> > > > > enumerates the device.
> > > >
> > > > Is this to say it is needed before the struct pci_host_bridge is 
> > > > created?
> > > >
> > > > > I originally added a separate find capability related
> > > > > function for CDNS in the following patch. It's also copied directly 
> > > > > from
> > > > > DWC.
> > > > > Mani felt there was too much duplicate code and also suggested 
> > > > > passing a
> > > > > callback function that could manipulate the registers of the root 
> > > > > port of
> > > > > DWC
> > > > > or CDNS.
> > > >
> > > > I very much like the direction this patchset is moving (moving shared
> > > > part of controllers code to core), I just feel this doesn't go far 
> > > > enough
> > > > when it's passing function pointer to the read function.
> > > >
> > > > I admit I've never written a controller driver so perhaps there's
> > > > something detail I lack knowledge of but I'd want to understand why
> > > > struct pci_ops (which exists both in pci_host_bridge and pci_bus) 
> > > > cannot
> > > > be used?
> > >
> > >
> > > I don't know if the following code can make it clear to you.
> > >
> > > static const struct dw_pcie_host_ops qcom_pcie_dw_ops = {
> > >     .host_init    = qcom_pcie_host_init,
> > >                    pcie->cfg->ops->post_init(pcie);
> > >                      qcom_pcie_post_init_2_3_3
> > >                        dw_pcie_find_capability(pci, PCI_CAP_ID_EXP);
> > > };
> > >
> > > int dw_pcie_host_init(struct dw_pcie_rp *pp)
> > >    bridge = devm_pci_alloc_host_bridge(dev, 0);
> >
> > It does this almost immediately:
> >
> >      bridge->ops = &dw_pcie_ops;
> >
> > Can we like add some function into those ops such that the necessary read
> > can be performed? Like .early_root_config_read or something like that?
> >
> > Then the host bridge capability finder can input struct pci_host_bridge
> > *host_bridge and can do 
> > host_bridge->ops->early_root_cfg_read(host_bridge,
> > ...). That would already be a big win over passing the read function
> > itself as a pointer.
> >
> > Hopefully having such a function in the ops would allow moving other
> > common controller driver functionality into PCI core as well as it would
> > abstract the per controller read function (for the time before everything
> > is fully instanciated).
> >
> > Is that a workable approach?
>
> I'll try to add and test it in your way first.
>
> Another problem here is that I've seen some drivers invoke 
> dw_pcie_find_*capability before if (pp->ops->init) {. When I confirm it, 
> or I'll see if I can cover all the issues.
>
> If I pass the test, I will provide the temporary patch here, please 
> check whether it is OK, and then submit the next version. If not, we'll 
> discuss it.

Hi Ilpo,

Another question comes to mind:
If working in EP mode, devm_pci_alloc_host_bridge will not be executed 
and there will be no struct pci_host_bridge.

Don't know if you have anything to add?

> Thank you very much for your advice.
>
> > >    if (pp->ops->host_init)
> > >      pp->ops = &qcom_pcie_dw_ops;  // qcom here needs to find capability
> > >
> > >    pci_host_probe(bridge); // pcie enumerate flow
> > >      pci_scan_root_bus_bridge(bridge);
> > >        pci_register_host_bridge(bridge);
> > >          bus->ops = bridge->ops;   // Only pci bus ops can be used

Best regards,
Hans