[PATCH 0/3] PCI: dwc: Cache PCIe capability offset and simplify drivers

[Manivannan Sadhasivam]

On Wed, May 20, 2026 at 12:27:21AM +0800, Hans Zhang wrote:
> 
> 
> On 5/20/26 00:15, Manivannan Sadhasivam wrote:
> > On Wed, May 20, 2026 at 12:09:28AM +0800, Hans Zhang wrote:
> > > 
> > > 
> > > On 5/19/26 21:57, Manivannan Sadhasivam wrote:
> > > > On Sat, May 09, 2026 at 09:51:49PM +0800, Hans Zhang wrote:
> > > > > The DWC PCIe core and its many platform drivers repeatedly call
> > > > > dw_pcie_find_capability(pci, PCI_CAP_ID_EXP) to obtain the offset of the
> > > > > PCI Express Capability structure. This is wasteful and makes the code
> > > > > verbose. And some even search for the PCI_CAP_ID_EXP offset value within
> > > > > the suspend/resume functions.
> > > > > 
> > > > 
> > > > Sashiko has flagged some real issues with this series in accessing DBI space
> > > > very early and 'pci->pcie_cap' being 0.
> > > 
> > > 
> > > Hi Mani,
> > > 
> > > We have discussed this issue in the Cadence driver. I think it won't cause
> > > any problems. Specifically as follows:
> > > 
> > > https://lore.kernel.org/linux-pci/5823faec-d972-4c77-90ec-a215c686e0a8@163.com/
> > > """
> > > As per PCIe r7.0, sec 7.5.1.1.11, Since all PCI Express Functions are
> > > required to implement the PCI Express Capability structure, which
> > > must be included somewhere in this linked list.
> > > """
> > > 
> > > 
> > > 
> > > Bjorn also responded as follows:
> > > https://lore.kernel.org/linux-pci/20260505212306.GA744158@bhelgaas/
> > > """
> > > It's true that all Root Ports must have a PCIe Capability.
> > > """
> > > 
> > 
> > Ok, what about reading the DBI registers very early?
> 
> Hi Mani,
> 
> Yes. I have performed the DBI read register operation at the very beginning
> of the following code.
> 
> 
> dw_pcie_ep_init()
>   dw_pcie_get_pcie_cap(pci);
> 
> dw_pcie_host_init
>   dw_pcie_get_pcie_cap(pci);
> 

These both calls will cause crash on a lot of platforms because these will be
reading the DBI registers while the resources are not enabled.

- Mani

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