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SubjectRe: [PATCH v3] PCI: aardvark: Don't touch PCIe registers if no card connected
On Thu, Jul 02, 2020 at 10:30:36AM +0200, Pali Rohár wrote:
> When there is no PCIe card connected and advk_pcie_rd_conf() or
> advk_pcie_wr_conf() is called for PCI bus which doesn't belong to emulated
> root bridge, the aardvark driver throws the following error message:
>
> advk-pcie d0070000.pcie: config read/write timed out
>
> Obviously accessing PCIe registers of disconnected card is not possible.
>
> Extend check in advk_pcie_valid_device() function for validating
> availability of PCIe bus. If PCIe link is down, then the device is marked
> as Not Found and the driver does not try to access these registers.
>
> This is just an optimization to prevent accessing PCIe registers when card
> is disconnected. Trying to access PCIe registers of disconnected card does
> not cause any crash, kernel just needs to wait for a timeout. So if card
> disappear immediately after checking for PCIe link (before accessing PCIe
> registers), it does not cause any problems.
>
> Signed-off-by: Pali Rohár <pali@kernel.org>
>
> ---
> Changes in V3:
> * Add comment to the code
> Changes in V2:
> * Update commit message, mention that this is optimization
> ---
> drivers/pci/controller/pci-aardvark.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/drivers/pci/controller/pci-aardvark.c b/drivers/pci/controller/pci-aardvark.c
> index 90ff291c24f0..d18f389b36a1 100644
> --- a/drivers/pci/controller/pci-aardvark.c
> +++ b/drivers/pci/controller/pci-aardvark.c
> @@ -644,6 +644,13 @@ static bool advk_pcie_valid_device(struct advk_pcie *pcie, struct pci_bus *bus,
> if ((bus->number == pcie->root_bus_nr) && PCI_SLOT(devfn) != 0)
> return false;
>
> + /*
> + * If the link goes down after we check for link-up, nothing bad
> + * happens but the config access times out.
> + */
> + if (bus->number != pcie->root_bus_nr && !advk_pcie_link_up(pcie))
> + return false;
> +
> return true;
> }

Question: this basically means that you can only effectively enumerate
bus number == root_bus_nr and AFAICS if at probe the link did not
come up it will never do, will it ?

Isn't this equivalent to limiting the bus numbers the bridge is capable
of handling ?

Reworded: if in advk_pcie_setup_hw() the link does not come up, what's
the point of trying to enumerate the bus hierarchy below the root bus ?

Thanks,
Lorenzo

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