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.

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 v2:
- Make the request GPIO optional.
- Correct the reset sequence as per PERST#
- Update commit message
---
 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..aa0c61d30049 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_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);
+
+	/*
+	 * 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, 0);
+
 	bridge = devm_pci_alloc_host_bridge(dev, sizeof(*port));
 	if (!bridge)
 		return -ENODEV;
-- 
2.44.1

On Wed, Feb 26, 2025 at 06:13:58PM +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.
> 
> 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 v2:
> - Make the request GPIO optional.
> - Correct the reset sequence as per PERST#
> - Update commit message
> ---
>  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..aa0c61d30049 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_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);
> +
> +	/*
> +	 * 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);

Use PCIE_T_PVPERL_MS.  The comment at that #define is probably enough.

> +	/* Deassert the reset signal */
> +	gpiod_set_value(reset_gpio, 0);
> +
>  	bridge = devm_pci_alloc_host_bridge(dev, sizeof(*port));
>  	if (!bridge)
>  		return -ENODEV;
> -- 
> 2.44.1
>