Re: [PATCH RFC v4 9/9] arm64: dts: qcom: glymur: Wire PCIe3a/3b to shared Gen5x8 PHY

[Qiang Yu]

On Fri, Jul 10, 2026 at 03:25:37PM +0200, Konrad Dybcio wrote:
> On 7/10/26 3:11 PM, Qiang Yu wrote:
> > On Fri, Jul 10, 2026 at 10:44:20AM +0200, Konrad Dybcio wrote:
> >> On 7/10/26 4:19 AM, Qiang Yu wrote:
> >>> On Mon, Jun 29, 2026 at 11:20:07AM +0200, Konrad Dybcio wrote:
> >>>> On 6/29/26 7:05 AM, Qiang Yu wrote:
> >>>>> On Wed, Jun 17, 2026 at 01:19:49PM +0200, Konrad Dybcio wrote:
> >>>>>> On 5/19/26 7:47 AM, Qiang Yu wrote:
> >>>>>>> Glymur PCIe3 uses a single shared Gen5x8 QMP PHY block. Model PCIe3a and
> >>>>>>> PCIe3b as consumers of that shared PHY provider instead of separate PHY
> >>>>>>> nodes.
> 
> [...]
> 
> >>>>>>> +			linux,pci-domain = <3>;
> >>>>>>> +			num-lanes = <8>;
> >>>>>>
> >>>>>> Is it fine to keep num-lanes 8 here even for configurations with
> >>>>>> bifurcated PHY?
> >>>>>>
> >>>>>> I would assume so, given essentially this is a x8 host, whose 4
> >>>>>> lanes may simply be effectively NC 
> >>>>>>
> >>>>> Actually, on existing platforms, the PCIe3a and PCIe3b controllers are
> >>>>> never enabled at the same time. When PCIe3a is exposed, it is always in an
> >>>>> x8 slot. But if we have a x4+x4 platform in future, we can simply override
> >>>>> num-lanes to 4 in the board.dts.
> >>>>
> >>>> My question is whether that will be necessary - if yes, sure, we
> >>>> can do it, but if not, we can conclude on this early and not have
> >>>> to fight over it in a couple months
> >>>>
> >>> I think we do need to override it in that case. If both PCIe3a and PCIe3b
> >>> are enabled in x4+x4 mode but PCIe3a keeps num-lanes = <8>, userspace
> >>> will see an 8-lane slot. If an x8-capable EP is connected to that slot,
> >>> both ends will advertise x8 support, but the link is up at x4. That looks
> >>> like a genuine bug from the user's point of view.
> >>
> >> Do we know what's advertised on x86 PCs with bifurcated lanes?
> >>
> > On QCB, it advertise x8. On CRD, PCIe3a is hidden.
> 
> No no, I meant to ask whether we know what the userspace gets to see on
> e.g. an AMD system with a bifurcated slot. I would expect that LnkCap
> may report a different value, but not necessarily. I would assume we
> should probably mimic whatever that does, unless the spec takes a
> stance on what should be done.
>
I don't have any AMD/Intel hardware with a bifurcated slot at hand,
and I couldn't find an lspci example showing what LnkCap reports in
that case either, so I can't confirm what x86 does here.

But can you elaborate on what is the benefits to set it to 8 in x4+x4
mode?

- Qiang Yu