Re: [PATCH v2] PCI/sysfs: document the link speed and width attributes

On Thu, Aug 20, 2026 at 10:03:53PM +0200, Ferran Duarri wrote:
> max_link_speed, max_link_width, current_link_speed and current_link_width
> have been exported under /sys/bus/pci/devices/.../ since 2018, by
> commit 56c1af4606f0 ("PCI: Add sysfs max_link_speed/width, current_link_speed/width, etc"),
> and none of the four appear anywhere in Documentation/ABI.
> 
> The gap matters most for current_link_speed. current_link_speed_show()
> performs a fresh PCI_EXP_LNKSTA read on every open, so the value reflects
> the link state at that instant. Modern GPUs retrain their link continuously
> as part of idle power management, which means a single read can legitimately
> return any speed the link supports, not the speed the link will use under
> load.
> 
> Observed on an RTX 5070 in a PCIe 4.0 x16 slot, same boot, no configuration
> change between the two reads: 5.0 GT/s while idle, 16.0 GT/s under load.
> Comparing current_link_speed against max_link_speed at idle is therefore not
> a valid test for a degraded link, though it reads like one.
> 
> Document all four. For the max_* pair, state that each reports the
> capability of the device it is read from and not a property of the link: a
> link trains at the lower of what its two ends support, so an endpoint
> capable of more than the port above it reports the higher figure while that
> port reports the lower one. Record where each value comes from, which
> differs between the two attributes. max_link_speed is derived from the
> Supported Link Speeds Vector in Link Capabilities 2, capped by Max Link
> Speed in Link Capabilities, synthesized from the latter alone on devices
> predating PCIe r3.0, and cached at enumeration. max_link_width is read from
> Maximum Link Width in Link Capabilities on each access.
> 
> For current_link_speed, state that it is instantaneous, that comparing it
> against max_link_speed at idle is not a valid degradation test, and that
> callers wanting what the link will actually deliver should sample under
> load -- noting that max_link_speed is not that figure either, being one
> end's capability rather than the link's.
> 
> No functional change.
> 
> Signed-off-by: Ferran Duarri <[email protected]>

Did you forget the Assisted-by: tag?

> ---
> Changes in v2, all corrections to what v1 claimed rather than new material:

You sent 2 v2 patches :(