Re: [PATCH v5 7/9] PCI: PCI: Add pcie_link_is_active() to determine if the PCIe link is active

[Krishna Chaitanya Chundru]

On 4/13/2025 10:44 PM, Lukas Wunner wrote:
> On Sat, Apr 12, 2025 at 05:52:56AM +0200, Lukas Wunner wrote:
> > On Sat, Apr 12, 2025 at 07:19:56AM +0530, Krishna Chaitanya Chundru wrote:
> > > Introduce a common API to check if the PCIe link is active, replacing
> > > duplicate code in multiple locations.
> > >
> > > Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@xxxxxxxxxxxxxxxx>
> >
> > Reviewed-by: Lukas Wunner <lukas@xxxxxxxxx>
>
> Looking at this with a fresh pair of eyeballs, I realize there's an issue
> here, so unfortunately I have to retract the Reviewed-by:
>
> pcie_link_is_active() differs from the existing pciehp_check_link_active()
> in that it returns 0 not only if the link is down, but also if the
> Config Space read returns with an error.
>
> In particular, if Config Space of a hotplug bridge is inaccessible,
> 0 is returned instead of -ENODEV with this patch.  That can happen if
> the hotplug bridge itself has been hot-removed, which is common with
> Thunderbolt, but also on servers with nested PCIe switches.
>
> The existing invocations of pciehp_check_link_active() do the right
> thing if the hotplug bridge has been hot-removed, but after this patch
> they no longer do.  For example in this hunk ...
>
> > > --- a/drivers/pci/hotplug/pciehp_hpc.c
> > > +++ b/drivers/pci/hotplug/pciehp_hpc.c
> > > @@ -584,7 +557,7 @@ static void pciehp_ignore_dpc_link_change(struct controller *ctrl,
> > >   	 * Synthesize it to ensure that it is acted on.
> > >   	 */
> > >   	down_read_nested(&ctrl->reset_lock, ctrl->depth);
> > > -	if (!pciehp_check_link_active(ctrl))
> > > +	if (!pcie_link_is_active(ctrl_dev(ctrl)))
> > >   		pciehp_request(ctrl, PCI_EXP_SLTSTA_DLLSC);
> > >   	up_read(&ctrl->reset_lock);
> > >   }
>
> ... pciehp_request() will be called if the hotplug bridge was
> hot-removed, which isn't the right thing to do.  The current
> behavior is to do nothing.
>
> I realize I steered you in the wrong direction because in my
> review of your v4 I asked why pcie_link_is_active() doesn't
> return a bool:
>
> https://lore.kernel.org/all/Z72TRBvpzizcgm9S@xxxxxxxxx/
>
> So I sincerely apologize for that!  You actually did the right
> thing in v4 by returning a negative int if the Config Space read
> returned an error.

Ok, No problem. I will revert v4 patch i.e returning int instead of
bool. I will wait for few days for any other new review comments.

- Krishna Chaitanya.
> Thanks,
>
> Lukas