Re: [PATCH v3 2/2] PCI: xilinx-cpm: Add support for PCIe RP PERST# signal

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

On Thu, Feb 27, 2025 at 09:54:54AM +0530, Sai Krishna Musham wrote:
> Add GPIO-based control for the PCIe Root Port PERST# signal.
> 
> According to section 2.2 of the PCIe Electromechanical Specification
> (Revision 6.0), PERST# signal has to be deasserted after a delay of
> 100 ms (T_PVPERL) to ensure proper reset sequencing during PCIe
> initialization.
> 
> Adapt to use the GPIO framework and make reset optional to keep DTB
> backward compatibility.
> 
> Signed-off-by: Sai Krishna Musham <sai.krishna.musham@amd.com>
> ---
> This patch depends on the following patch series.
> https://lore.kernel.org/all/20250217072713.635643-3-thippeswamy.havalige@amd.com/
> 
> Changes for v3:
> - Use PCIE_T_PVPERL_MS define.
> 
> Changes for v2:
> - Make the request GPIO optional.
> - Correct the reset sequence as per PERST#
> - Update commit message
> ---
>  drivers/pci/controller/pcie-xilinx-cpm.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/drivers/pci/controller/pcie-xilinx-cpm.c b/drivers/pci/controller/pcie-xilinx-cpm.c
> index 81e8bfae53d0..558f1d602802 100644
> --- a/drivers/pci/controller/pcie-xilinx-cpm.c
> +++ b/drivers/pci/controller/pcie-xilinx-cpm.c
> @@ -6,6 +6,8 @@
>   */
>  
>  #include <linux/bitfield.h>
> +#include <linux/delay.h>
> +#include <linux/gpio/consumer.h>
>  #include <linux/interrupt.h>
>  #include <linux/irq.h>
>  #include <linux/irqchip.h>
> @@ -568,8 +570,24 @@ static int xilinx_cpm_pcie_probe(struct platform_device *pdev)
>  	struct device *dev = &pdev->dev;
>  	struct pci_host_bridge *bridge;
>  	struct resource_entry *bus;
> +	struct gpio_desc *reset_gpio;
>  	int err;
>  
> +	/* Request the GPIO for PCIe reset signal */
> +	reset_gpio = devm_gpiod_get_optional(dev, "reset", GPIOD_OUT_HIGH);
> +	if (IS_ERR(reset_gpio)) {
> +		dev_err(dev, "Failed to request reset GPIO\n");
> +		return PTR_ERR(reset_gpio);
> +	}
> +
> +	/* Assert the reset signal */
> +	gpiod_set_value(reset_gpio, 1);
> +
> +	msleep(PCIE_T_PVPERL_MS);
> +
> +	/* Deassert the reset signal */
> +	gpiod_set_value(reset_gpio, 0);
> +

You should deassert the PERST# only after the power and refclk are stable. Even
though this driver is not initializing any resources, it makes sense to move the
assert + deassert logic at the very end of xilinx_cpm_pcie_init_port() as this
function sounds like the once initializing the PCIe port.

- Mani

-- 
மணிவண்ணன் சதாசிவம்