linux-kernel - Re: [PATCHv4 2/2] PCI: fu740: Force gen1 for initial device probe

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Date:   Mon, 28 Feb 2022 23:15:39 +0000
From:   Ben Dooks <ben.dooks@...ethink.co.uk>
To:     "Maciej W. Rozycki" <macro@...am.me.uk>,
        Bjorn Helgaas <helgaas@...nel.org>
Cc:     paul.walmsley@...ive.com, greentime.hu@...ive.com,
        lorenzo.pieralisi@....com, robh@...nel.org, kw@...ux.com,
        bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org
Subject: Re: [PATCHv4 2/2] PCI: fu740: Force gen1 for initial device probe

On 23/02/2022 21:19, Maciej W. Rozycki wrote:
> On Wed, 23 Feb 2022, Bjorn Helgaas wrote:
> 
>>> +	dw_pcie_dbi_ro_wr_dis(dw);
>>> +}
>>> +
>>>   static int fu740_pcie_start_link(struct dw_pcie *pci)
>>>   {
>>>   	struct device *dev = pci->dev;
>>>   	struct fu740_pcie *afp = dev_get_drvdata(dev);
>>>   
>>> +	/* Force PCIe gen1 otherwise Unmatched board does not probe */
>>> +	fu740_pcie_force_gen1(pci, afp);
>>
>> I guess the "Unmatched" board is the only thing we need to care about
>> here?  Are there or will there be other boards that don't need this?
> 
>   I wonder if this is the other side of a supposed erratum observed here:
> 
> <https://lore.kernel.org/all/alpine.DEB.2.21.2202010240190.58572@angie.orcam.me.uk/>
> 
> Downstream there's the same ASMedia ASM2824 PCIe switch whose downstream
> ports don't want to train with a Pericom part above Gen1.
> 
>   Of course we don't know an ASM2824 is there until we have successfully
> negotiated the link, so we may have to generalise my proposal if we can
> find a way similar to what I have done for U-boot that does not disturb
> Linux's operation.  This is because there are PCIe option cards out there
> with the ASM2824 onboard, so it could be possible for the problem to
> trigger anywhere where the conditions for the erratum are met.
> 
>   Also in that case retraining should work with the cap removed to get a
> higher final speed just as with the Pericom part.

Possibly. I have just been working on a patch to better force Gen1
speeds and then restore the config which is working on my Unmatched
board.


-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

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