Re: [PATCH] PCI: rockchip: Support quirk to disable 5 GT/s (PCIe 2.x) link rate

[Shawn Lin]

Hi Brain,

å 2016/9/23 9:15, Brian Norris åé:
> Hi Shawn,
>
> On Fri, Sep 23, 2016 at 08:27:35AM +0800, Shawn Lin wrote:
> > å 2016/9/23 1:31, Brian Norris åé:
> > > rk3399 supports PCIe 2.x link speeds marginally at best, and on some
> > > boards, the link won't train at 5 GT/s at all. Rather than sacrifice 500
> > > ms waiting for training that will never happen, let's support a device
> > > tree quirk flag to disable generation 2 speeds entirely.
> >
> > I was thinking about could we get target link speed [TLS] from the
> > end-point when finishing Gen1 training, but it seems that the location
> > of ep's TLS is not fixed.
>
> Indeed it's not, but we could probably handle that if absolutely needed
> (get a reference to the root port pci_dev somehow, then use the existing
> helpers to walk children and get the computed ->pcie_cap offset). But

Right, we could probably walk through the ep's cap and get this, but
sure, it's not the problem here, and that is maybe what I want to dig
more later.

Thanks for sharing this.

> that's not the problem here; we have 5 GT/s devices, but they are not
> running at 5 GT/s because link training can't pass. We have been told
> there are still SI issues, and so you wouldn't really be able to turn
> this out at runtime anyway.
>
> But sure, I suppose that'd be a way to (for chips/boards that don't have
> SI issues) determine whether or not to attempt gen2 training at all.
> That does sound better than just timing out after 500ms...
>
> > Anyway, your patch looks sane to me as we leave gen2 as default and
> > people could drop that feature by adding rockchip,disable-gen2 to
> > their dts if they are sure the board would never supoort Gen2 devices.
> >
> > Acked-by: Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>
>
> Thanks.
>
> Brian


--
Best Regards
Shawn Lin