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

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

On Mon, Feb 24, 2025 at 12:00:46PM +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 (TPVPERL) to ensure proper reset sequencing during PCIe
> initialization.
> 
> 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/
> ---
>  drivers/pci/controller/pcie-xilinx-cpm.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/drivers/pci/controller/pcie-xilinx-cpm.c b/drivers/pci/controller/pcie-xilinx-cpm.c
> index 81e8bfae53d0..0e31b85658e6 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 +570,29 @@ 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(dev, "reset", GPIOD_OUT_LOW);

You've defined the polarity as 0x1 in the binding. Which corresponds to
GPIO_ACTIVE_LOW. So if you request the GPIO as GPIOD_OUT_LOW, it means the host
is going to drive the PERST# line as 'high', which corresponds to PERST#
deassert. I don't think you'd want that and if that is what is really happening,
the endpoint state machine would be broken. So I suspect that the polarity of
your PERST# line is wrong.

- Mani

> +	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, 0);
> +
> +	/*
> +	 * As per section 2.2 of the PCI Express Card Electromechanical
> +	 * Specification (Revision 6.0), the deassertion of the PERST# signal
> +	 * should be delayed by 100 ms (TPVPERL).
> +	 */
> +	msleep(100);
> +
> +	/* Deassert the reset signal */
> +	gpiod_set_value(reset_gpio, 1);
> +
>  	bridge = devm_pci_alloc_host_bridge(dev, sizeof(*port));
>  	if (!bridge)
>  		return -ENODEV;
> -- 
> 2.44.1
> 

-- 
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