Re: [PATCH v2 2/4] PCI: qcom: Do not enumerate bus before endpoint devices are ready

[Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>]

On 5/6/2025 1:09 PM, Niklas Cassel wrote:
> Commit 36971d6c5a9a ("PCI: qcom: Don't wait for link if we can detect Link
> Up") changed so that we no longer call dw_pcie_wait_for_link(), and instead
> enumerate the bus when receiving a Link Up IRQ.
> 
> Before 36971d6c5a9a, we called dw_pcie_wait_for_link(), and in the first
> iteration of the loop, the link will never be up (because the link was just
> started), dw_pcie_wait_for_link() will then sleep for LINK_WAIT_SLEEP_MS
> (90 ms), before trying again.
> 
> This means that even if a driver was missing a msleep(PCIE_T_RRS_READY_MS)
> (100 ms), because of the call to dw_pcie_wait_for_link(), enumerating the
> bus would essentially be delayed by that time anyway (because of the sleep
> LINK_WAIT_SLEEP_MS (90 ms)).
> 
> While we could add the msleep(PCIE_T_RRS_READY_MS) after deasserting PERST
> (qcom already has an unconditional 1 ms sleep after deasserting PERST),
> that would essentially bring back an unconditional delay during probe (the
> whole reason to use a Link Up IRQ was to avoid an unconditional delay
> during probe).
> 
> Thus, add the msleep(PCIE_T_RRS_READY_MS) before enumerating the bus in the
> IRQ handler. This way, for qcom SoCs that has a link up IRQ, we will not
> have a 100 ms unconditional delay during boot for unpopulated PCIe slots.
> 
> Fixes: 36971d6c5a9a ("PCI: qcom: Don't wait for link if we can detect Link Up")
> Signed-off-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
> ---
>   drivers/pci/controller/dwc/pcie-qcom.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/pci/controller/dwc/pcie-qcom.c b/drivers/pci/controller/dwc/pcie-qcom.c
> index dc98ae63362d..01a60d1f372a 100644
> --- a/drivers/pci/controller/dwc/pcie-qcom.c
> +++ b/drivers/pci/controller/dwc/pcie-qcom.c
> @@ -1565,6 +1565,7 @@ static irqreturn_t qcom_pcie_global_irq_thread(int irq, void *data)
>   
>   	if (FIELD_GET(PARF_INT_ALL_LINK_UP, status)) {
>   		dev_dbg(dev, "Received Link up event. Starting enumeration!\n");
> +		msleep(PCIE_T_RRS_READY_MS);
>   		/* Rescan the bus to enumerate endpoint devices */
>   		pci_lock_rescan_remove();
>   		pci_rescan_bus(pp->bridge->bus);