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

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Date:   Thu, 22 Sep 2016 18:15:19 -0700
From:   Brian Norris <briannorris@...omium.org>
To:     Shawn Lin <shawn.lin@...k-chips.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, Jeffy Chen <jeffy.chen@...k-chips.com>,
        Wenrui Li <wenrui.li@...k-chips.com>,
        Heiko Stuebner <heiko@...ech.de>, linux-pci@...r.kernel.org,
        linux-rockchip@...ts.infradead.org
Subject: Re: [PATCH] PCI: rockchip: Support quirk to disable 5 GT/s (PCIe
 2.x) link rate

Hi Shawn,

On Fri, Sep 23, 2016 at 08:27:35AM +0800, Shawn Lin wrote:
> 在 2016/9/23 1:31, Brian Norris 写道:
> >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
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@...k-chips.com>

Thanks.

Brian