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

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Date:   Fri, 3 Dec 2021 22:47:08 +0900
From:   Hector Martin <marcan@...can.st>
To:     Rob Herring <robh@...nel.org>
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 02/12/2021 23.33, Rob Herring wrote:
>>>>>> +       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.
>>
>> I thought the PCIE_LINK_WIDTH_SPEED_CONTROL thing had to be set later,
>> but trying it now I realized we were missing a bit of initialization
>> that was causing it not to work. Indeed it can be done there and we can
>> drop it from the kernel.
>>
>> We could even do the max-link-speed thing in m1n1 if we want. It has
>> access to the value from the ADT directly, which to be correct we'd have
>> to dynamically transplant to the DT, since there's at least one device
>> that has different PCIe devices on one port depending on hardware
>> variant, while sharing a devicetree. If we're okay with the kernel just
>> not implementing this feature for now, we can say it's the bootloader's job.
>>
>> Ultimately we ship the DTs along with m1n1, so there's an argument that
>> if some day we need to override the max-link-speed for whatever reason
>> over what the ADT says, well, we'd be shipping the updated DT along with
>> m1n1 anyway, so we might as well make m1n1 do it... if so, it might make
>> sense to drop those properties from the actual DTs we ship altogether,
>> at least for now.
>>
>> If we decide to make it m1n1's job entirely, we can drop this patch
>> altogether, at least for now (I can't say how this will interact with
>> suspend/resume and other power management, and hotplug... but we'll open
>> that can of worms when we get there).
> 
> Shouldn't you be setting PCI_EXP_LNKCAP_SLS and/or PCI_EXP_LNKCAP2 if
> you need to limit the max speed and then you can use that instead of
> max-link-speed? If that's lost in low power modes, the driver just has
> to save and restore it.

Those registers aren't writable as far as I can tell. All we can do is 
set LNKCTL2 to tell the hardware what actual max speed to use, the same 
thing this patch does.

-- 
Hector Martin (marcan@...can.st)
Public Key: https://mrcn.st/pub