linux-kernel - Re: [PATCH] PCI: apple: Configure link speeds properly

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Date:   Mon, 29 Nov 2021 09:02:18 -0600
From:   Rob Herring <robh@...nel.org>
To:     Hector Martin <marcan@...can.st>
Cc:     Marc Zyngier <maz@...nel.org>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        PCI <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PCI: apple: Configure link speeds properly

On Wed, Nov 24, 2021 at 1:40 AM Hector Martin <marcan@...can.st> wrote:
>
> On 24/11/2021 11.23, Rob Herring wrote:
> >> +#include "../pci.h"
> >> +/* Apple PCIe is based on DesignWare IP and shares some registers */
> >> +#include "dwc/pcie-designware.h"
> >
> > I'm starting to regret this not being part of the DWC driver...
>
> Main issue is the DWC driver seems to have a pretty hard-coded
> assumption of one port per controller, plus does a bunch of stuff
> differently for the higher layers. It seems Apple used the DWC PHY/LTSSM
> bits, then rolled their own upper layer.

Yeah, it would need some work...

> >> +/* The offset of the PCIe capabilities structure in bridge config space */
> >> +#define PCIE_CAP_BASE          0x70
> >
> > This offset is discoverable, so don't hardcode it.
>
> Sure, it just means I have to reinvent the PCI capability lookup wheel
> again. I'd love to use the regular accessors, but the infrastructure
> isn't up to the point where we can do that yet yere. DWC also reinvents
> this wheel, but we can't reuse that code because it pokes these
> registers through a separate reg range, not config space (even though it
> seems like they should be the same thing? I'm not sure what's going on
> in the DWC devices... for the Apple controller it's just the ECAM).

Since it is just ECAM, can you use the regular config space accessors?

> >> +       max_gen = of_pci_get_max_link_speed(port->np);
> >> +       if (max_gen < 0) {
> >> +               dev_err(port->pcie->dev, "max link speed not specified\n");
> >
> > Better to fail than limp along in gen1? Though you don't check the
> > return value...
> >
> > Usually, the DT property is there to limit the speed when there's a
> > board limitation.
>
> The default *setting* is actually Gen4, but without
> PCIE_LINK_WIDTH_SPEED_CONTROL poked it always trains at Gen1. Might make
> more sense to only set the LNKCTL field if max-link-speed is specified,
> and unconditionally poke that bit. That'll get us Gen4 by default (or
> even presumably Gen5 in future controllers, if everything else stays
> compatible).

You already do some setup in firmware for ECAM, right? I think it
would be better if you can do any default setup there and then
max-link-speed is only an override for the kernel.

Rob