Re: [PATCH v2] PCI/PTM: Do not enable PTM solely based on the capability existense

[Lukas Wunner <lukas@wunner.de>]

On Tue, Oct 28, 2025 at 07:04:27AM +0100, Mika Westerberg wrote:
> It is not advisable to enable PTM solely based on the fact that the
> capability exists. Instead there are separate bits in the capability
> register that need to be set for the feature to be enabled for a given
> component (this is suggestion from Intel PCIe folks):
> 
>   - PCIe Endpoint that has PTM capability must to declare requester
>     capable
>   - PCIe Switch Upstream Port that has PTM capability must declare
>     at least responder capable
>   - PCIe Root Port must declare root port capable.
[...]
> This happens because Linux sees the PTM capability and blindly enables
> PTM which then causes the AER error to trigger.
> 
> Fix this by enabling PTM only if the above described criteria is met.
> 
> Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>

Reviewed-by: Lukas Wunner <lukas@wunner.de>

A stable designation might be merited, it looks like we've been doing
this wrong since forever:

Fixes: 9bb04a0c4e26 ("PCI: Add Precision Time Measurement (PTM) support")
Cc: stable@vger.kernel.org  # v4.9+

A spec reference in the commit message may also be helpful:

PCIe r7.0 sec 6.21.1 figure 6-21

I guess Bjorn could add those when applying if he deems them necessary,
so probably no reason to respin just for that.

Thanks,

Lukas