linux-kernel - Re: [PATCH] PCI: dwc: Add the sysfs property to provide the LTSSM status of the PCIe link

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Message-ID: <20250123164944.GA1223935@bhelgaas>
Date: Thu, 23 Jan 2025 10:49:44 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Hans Zhang <18255117159@....com>
Cc: jingoohan1@...il.com, manivannan.sadhasivam@...aro.org,
	lpieralisi@...nel.org, kw@...ux.com, robh@...nel.org,
	bhelgaas@...gle.com, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, rockswang7@...il.com
Subject: Re: [PATCH] PCI: dwc: Add the sysfs property to provide the LTSSM
 status of the PCIe link

On Thu, Jan 23, 2025 at 03:13:26PM +0800, Hans Zhang wrote:
> Add the sysfs property to provide a view of the current link's LTSSM
> status from the root port device.
> 
>   /sys/bus/pci/devices/<dev>/ltssm_status

Would need a rationale, i.e., what benefit this provides.  Obviously
this is currently only implemented for DWC-based controllers and
probably not ever available for ACPI or other generic host bridges.

Also documentation somewhere in
Documentation/ABI/testing/sysfs-bus-pci*.

LTSSM is applicable to all Downstream Ports, including both Root Ports
and Switch Downstream Ports, but in general I doubt this information
is available for Switches in any generic way.

> +++ b/drivers/pci/pci-sysfs.c
> @@ -1696,6 +1696,9 @@ const struct attribute_group *pci_dev_attr_groups[] = {
>  #endif
>  #ifdef CONFIG_PCIEASPM
>  	&aspm_ctrl_attr_group,
> +#endif
> +#ifdef CONFIG_PCIE_DW_HOST
> +	&dw_ltssm_status_attr_group,
>  #endif

I'm not convinced of the value of potentially dozens of device- or
vendor-specific additions like this.

Bjorn