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

On 5/20/26 00:49, Manivannan Sadhasivam wrote:
> 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/[email protected]/
>>>> """
>>>> 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.

Hi Mani,

Next, I will check all the DBI register-related issues. It will be fixed 
in the next version. Thank you very much.

Best regards,
Hans