Re: [PATCH v4 03/11] PCI/ACPI: Add PERST# Assertion Delay _DSM method

[Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@linux.intel.com>]

On 5/29/25 4:16 AM, Badal Nilawar wrote:
> From: Anshuman Gupta <anshuman.gupta@intel.com>
>
> Implement _DSM Method 0Bh as per PCI firmware specs
> section 4.6.11 Rev 3.3.
>
> Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
> Signed-off-by: Badal Nilawar <badal.nilawar@intel.com>
> ---
>   drivers/pci/pci-acpi.c   | 57 ++++++++++++++++++++++++++++++++++++++++
>   include/linux/pci-acpi.h |  8 +++++-
>   2 files changed, 64 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c
> index d33efba4ca94..88044491feaa 100644
> --- a/drivers/pci/pci-acpi.c
> +++ b/drivers/pci/pci-acpi.c
> @@ -1531,6 +1531,63 @@ int pci_acpi_request_d3cold_aux_power(struct pci_dev *dev, u32 requested_power,
>   }
>   EXPORT_SYMBOL_GPL(pci_acpi_request_d3cold_aux_power);
>   
> +/**
> + * pci_acpi_add_perst_assertion_delay - Request PERST# delay via ACPI DSM
> + * @dev: PCI device instance
> + * @delay_us: Requested delay_us
> + *
> + * This function sends a request to the host BIOS via ACPI _DSM to grant the
> + * required PERST# delay for the specified PCI device. It evaluates the _DSM
> + * to request the PERST# delay and handles the response accordingly.
> + *
> + * Return: returns 0 on success and errno on failure.
> + */
> +int pci_acpi_add_perst_assertion_delay(struct pci_dev *dev, u32 delay_us)
> +{
> +	union acpi_object in_obj = {
> +		.integer.type = ACPI_TYPE_INTEGER,
> +		.integer.value = delay_us,
> +	};
> +
> +	union acpi_object *out_obj;
> +	acpi_handle handle;
> +	int result, ret = -EINVAL;
> +
> +	if (!dev)
> +		return -EINVAL;
> +
> +	handle = ACPI_HANDLE(&dev->dev);
> +	if (!handle)
> +		return -EINVAL;
> +
> +	if (!acpi_check_dsm(handle, &pci_acpi_dsm_guid, 4, 1 << DSM_PCI_PERST_ASSERTION_DELAY)) {
> +		pci_dbg(dev, "ACPI _DSM 0%Xh not supported\n", DSM_PCI_PERST_ASSERTION_DELAY);
> +		return -ENODEV;
> +	}
> +
> +	out_obj = acpi_evaluate_dsm_typed(handle, &pci_acpi_dsm_guid, 4,
> +					  DSM_PCI_PERST_ASSERTION_DELAY,
> +					  &in_obj, ACPI_TYPE_INTEGER);
> +	if (!out_obj)
> +		return -EINVAL;
> +
> +	result = out_obj->integer.value;
> +
> +	if (result == delay_us) {
> +		pci_info(dev, "PERST# Assertion Delay set to %u microseconds\n", delay_us);
> +		ret = 0;

I think above is a debug message. If it is set properly, why would you want to know
the details?

> +	} else if (result == 0) {
> +		pci_warn(dev, "PERST# Assertion Delay request failed, no previous valid request\n");
> +	} else {
> +		pci_warn(dev, "PERST# Assertion Delay request failed, Previous valid delay: %u microseconds\n",
> +			 result);
> +	}

May be you don't need to elaborate the error details. Will following work?

pci_warn(dev, "PERST# Assertion Delay request failed, result:%u micro seconds\n", result);

> +
> +	ACPI_FREE(out_obj);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(pci_acpi_add_perst_assertion_delay);
> +
>   static void pci_acpi_set_external_facing(struct pci_dev *dev)
>   {
>   	u8 val;
> diff --git a/include/linux/pci-acpi.h b/include/linux/pci-acpi.h
> index 6079306ad754..e53d4893cf56 100644
> --- a/include/linux/pci-acpi.h
> +++ b/include/linux/pci-acpi.h
> @@ -122,6 +122,7 @@ extern const guid_t pci_acpi_dsm_guid;
>   #define DSM_PCI_POWER_ON_RESET_DELAY		0x08
>   #define DSM_PCI_DEVICE_READINESS_DURATIONS	0x09
>   #define DSM_PCI_D3COLD_AUX_POWER_LIMIT		0x0A
> +#define DSM_PCI_PERST_ASSERTION_DELAY		0x0B
>   
>   #ifdef CONFIG_PCIE_EDR
>   void pci_acpi_add_edr_notifier(struct pci_dev *pdev);
> @@ -135,7 +136,7 @@ int pci_acpi_set_companion_lookup_hook(struct acpi_device *(*func)(struct pci_de
>   void pci_acpi_clear_companion_lookup_hook(void);
>   int pci_acpi_request_d3cold_aux_power(struct pci_dev *dev, u32 requested_power,
>   				      u32 *retry_interval);
> -
> +int pci_acpi_add_perst_assertion_delay(struct pci_dev *dev, u32 delay_us);
>   #else	/* CONFIG_ACPI */
>   static inline void acpi_pci_add_bus(struct pci_bus *bus) { }
>   static inline void acpi_pci_remove_bus(struct pci_bus *bus) { }
> @@ -144,6 +145,11 @@ static inline int pci_acpi_request_d3cold_aux_power(struct pci_dev *dev, u32 req
>   {
>   	return -EOPNOTSUPP;
>   }
> +
> +static inline int pci_acpi_add_perst_assertion_delay(struct pci_dev *dev, u32 delay_us)
> +{
> +	return -EOPNOTSUPP;
> +}
>   #endif	/* CONFIG_ACPI */
>   
>   #endif	/* _PCI_ACPI_H_ */

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer