linux-kernel - Re: [PATCH v1 8/9] PCI: PLDA: starfive: Add JH7110 PCIe controller

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Date:   Thu, 3 Aug 2023 10:23:47 +0800
From:   Kevin Xie <kevin.xie@...rfivetech.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
CC:     Minda Chen <minda.chen@...rfivetech.com>,
        Daire McNamara <daire.mcnamara@...rochip.com>,
        Conor Dooley <conor@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Emil Renner Berthing <emil.renner.berthing@...onical.com>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-riscv@...ts.infradead.org>, <linux-pci@...r.kernel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        "Palmer Dabbelt" <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        "Philipp Zabel" <p.zabel@...gutronix.de>,
        Mason Huo <mason.huo@...rfivetech.com>,
        Leyfoon Tan <leyfoon.tan@...rfivetech.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        Pali Rohár <pali@...nel.org>,
        Marek Behún <kabel@...nel.org>
Subject: Re: [PATCH v1 8/9] PCI: PLDA: starfive: Add JH7110 PCIe controller



On 2023/8/3 1:18, Bjorn Helgaas wrote:
> On Tue, Aug 01, 2023 at 03:05:46PM +0800, Kevin Xie wrote:
>> On 2023/8/1 7:12, Bjorn Helgaas wrote:
>> ...
> 
>> > The delay required by sec 6.6.1 is a minimum of 100ms following exit
>> > from reset or, for fast links, 100ms after link training completes.
>> > 
>> > The comment at the call of advk_pcie_wait_for_link() [2] says it is
>> > the delay required by sec 6.6.1, but that doesn't seem right to me.
>> > 
>> > For one thing, I don't think 6.6.1 says anything about "link up" being
>> > the end of a delay.  So if we want to do the delay required by 6.6.1,
>> > "wait_for_link()" doesn't seem like quite the right name.
>> > 
>> > For another, all the *_wait_for_link() functions can return success
>> > after 0ms, 90ms, 180ms, etc.  They're unlikely to return after 0ms,
>> > but 90ms is quite possible.  If we avoided the 0ms return and
>> > LINK_WAIT_USLEEP_MIN were 100ms instead of 90ms, that should be enough
>> > for slow links, where we need 100ms following "exit from reset."
>> > 
>> > But it's still not enough for fast links where we need 100ms "after
>> > link training completes" because we don't know when training
>> > completed.  If training completed 89ms into *_wait_for_link(), we only
>> > delay 1ms after that.
>> 
>> That's the point, we will add a extra 100ms after PERST# de-assert
>> in the patch-v3 according to Base Spec r6.0 - 6.6.1:
>>         msleep(100);
>>         gpiod_set_value_cansleep(pcie->reset_gpio, 0);
>> 
>> +       /* As the requirement in PCIe base spec r6.0, system must wait a
>> +        * minimum of 100 ms following exit from a Conventional Reset
>> +        * before sending a Configuration Request to the device.*/
>> +       msleep(100);
>> +
>>         if (starfive_pcie_host_wait_for_link(pcie))
>>                 return -EIO;
> 
> For fast links (links that support > 5.0 GT/s), the 100ms starts
> *after* link training completes.  The above looks OK if starfive only
> supports slow links, but then I'm not sure why we would need
> starfive_pcie_host_wait_for_link().
> 
Yes, the maximum speed of JH7110 PCIe is 5.0 GT/s (Gen2x1).

About starfive_pcie_host_wait_for_link():
JH7110 SoC only has one root port in each PCIe controller (2 in total)
and they do not support hot-plug yet.
Thus, We add starfive_pcie_host_wait_for_link() to poll if it is a empty slot.
If nothing here, we will exit the probe() of this controller, and it will not
go into pci_host_probe() too.
This may not be a very standard logic, should we remove it or rewrite in a better way?

> Bjorn