Re: Re: [PATCH pci] PCI: don't skip probing entire device if first fn OF node has status = "disabled"

[陈华才]

+Cc Jianmin Lv

Hi, Jianmin,

You are the most familiar person in this field, could you please give some suggestions? Thank you.

Huacai


> -----原始邮件-----
> 发件人: "Vladimir Oltean" <vladimir.oltean@xxxxxxx>
> 发送时间:2023-06-02 06:15:32 (星期五)
> 收件人: "Bjorn Helgaas" <helgaas@xxxxxxxxxx>
> 抄送: linux-pci@xxxxxxxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, "Bjorn Helgaas" <bhelgaas@xxxxxxxxxx>, "Rob Herring" <robh@xxxxxxxxxx>, "Claudiu Manoil" <claudiu.manoil@xxxxxxx>, "Michael Walle" <michael@xxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, "Liu Peibao" <liupeibao@xxxxxxxxxxx>, "Binbin Zhou" <zhoubinbin@xxxxxxxxxxx>, "Huacai Chen" <chenhuacai@xxxxxxxxxxx>
> 主题: Re: [PATCH pci] PCI: don't skip probing entire device if first fn OF node has status = "disabled"
> 
> On Thu, Jun 01, 2023 at 12:51:39PM -0500, Bjorn Helgaas wrote:
> > > > Doing it in Linux would minimize dependences on the bootloader, so
> > > > that seems desirable to me. That means Linux needs to enumerate
> > > > Function 0 so it is visible to a driver or possibly a quirk.
> > > 
> > > Uhm... no, that wouldn't be enough. Only a straight revert would satisfy
> > > the workaround that we currently have for NXP ENETC in Linux.
> > 
> > I guess you mean a revert of 6fffbc7ae137?
> 
> Yes.
> 
> > This whole conversation is about whether we can rework 6fffbc7ae137 to
> > work both for Loongson and for you, so nothing is decided yet.
> 
> After reading
> https://lore.kernel.org/linux-pci/20221117020935.32086-1-liupeibao@xxxxxxxxxxx/
> and
> https://lore.kernel.org/linux-pci/20221103090040.836-1-liupeibao@xxxxxxxxxxx/
> and seeing the GMAC OF node at arch/mips/boot/dts/loongson/loongson64-2k1000.dtsi,
> I believe that a solution that would work for both Loongson and NXP would be to:
> 
> - patch loongson_dwmac_probe() to check for of_device_is_available()
> - revert commit 6fffbc7ae137 ("PCI: Honor firmware's device disabled
>   status")
> 
> I'm not sure what else of what was concretely proposed would work.
> Anything else is just wishful thinking that the PCI core can start
> enforcing a central policy, after letting device drivers get to choose
> how (and whether) to treat the "status" OF property for years on end.
> 
> As an added benefit, the disabled GMAC would become visible in lspci for
> the Loongson SoC.
> 
> > The point is, I assume you agree that it's preferable if we don't have
> > to depend on a bootloader to clear the memory.
> 
> I am confused by the message you are transmitting here.
> 
> With my user hat on, yes, maintaining the effect of commit 3222b5b613db
> from Linux is preferable.
> 
> Although Rob will probably not be happy about the way in which that will
> be achieved. And you haven't proposed ways in which that would remain
> possible, short of a revert of commit 6fffbc7ae137.
> 
> > After 6fffbc7ae137, the probe function is not called if the device is
> > disabled in DT because there's no pci_dev for it at all.
> 
> Correct, but commit 3222b5b613db pre-dates it by 2 years, and thus, it
> is broken by Rob's change.
> 
> > > My problem is that I don't really understand what was the functional
> > > need for commit 6fffbc7ae137 ("PCI: Honor firmware's device disabled
> > > status") in the first place, considering that any device driver can
> > > already fail to probe based on the same condition at its own will.
> > 
> > In general, PCI drivers shouldn't rely on DT.  If the bus driver (PCI
> > in this case) calls a driver's probe function, the driver can assume
> > the device exists.
> 
> Well, the device exists...
> 
> > But enetc is not a general-purpose driver, and if DT is the only way
> > to discover this property, I guess you're stuck doing that.
> 
> So what Loongson tried to do - break enumeration of the on-chip GMAC
> PCIe device at the level of the PCIe controller, if the GMAC's pinmuxing
> doesn't make it available for networking - is encouraged?
> 
> Do you consider that their patch would have been better in the original
> form, if instead of the "skip-scan" property, they would have built some
> smarts into drivers/pci/controller/pci-loongson.c which would intentionally
> break config space access to gmac@3,0, without requiring OF to specify this?
> 
> Are you saying that this "present but unusable due to pinmuxing" is an
> incorrect use of status = "disabled"? What would it constitute correct
> use of, then?
> 
> The analogous situation for ENETC would be to patch the "pci-host-ecam-generic"
> driver to read the SERDES and pinmuxing configuration of the SoC, and to
> mask/unmask the config access to function 0 based on that. I mean - I could...
> but is it really a good idea? The principle of separation of concerns
> tells me no. The fact that the pinmuxing of the device makes it unavailable
> pertains to the IP-specific logic, it doesn't change whether it's enumerable
> or accessible on its bus.
> 
> > > > Is DT the only way to learn the NXP SERDES configuration?  I think it
> > > > would be much better if there were a way to programmatically learn it,
> > > > because then you wouldn't have to worry about syncing the DT with the
> > > > platform configuration, and it would decouple this from the Loongson
> > > > situation.
> > > 
> > > Syncing the DT with the platform configuration will always be necessary,
> > > because for networking we will also need extra information which is
> > > completely non-discoverable, like a phy-handle or such, and that depends
> > > on the wiring and static pinmuxing of the SoC. So it is practically
> > > reasonable to expect that what is usable has status = "okay", and what
> > > isn't has status = "disabled". Not to mention, there are already device
> > > trees in circulation which are written that way, and those need to
> > > continue to work.
> > 
> > Just because we need DT for non-discoverable info A doesn't mean we
> > should depend on it for B if B *is* discoverable.
> 
> But the argument was: we already have device trees with a certain
> convention, and that is to expect having status = "disabled" for
> unusable ports. I don't believe that changing that is realistically in
> scope for fixing this. And if we have device trees with status =
> "disabled" in circulation which we (I) don't want to break, then we're
> back to square 1 regarding the probing of disabled devices.
> 
> > This question of disabling a device via DT but still needing to do
> > things to the device is ... kind of a sticky wicket.
> 
> It boils down to whether accessing a disabled device is permitted or
> not. I opened the devicetree specification and it didn't say anything
> conclusive. Though it's certainly above my pay grade to say anything
> with certainty in this area. Apart from "okay" and "disabled", "status"
> takes other documented values too, like "reserved", "fail" and
> "fail-sss". Linux treats everything that's not "okay" the same.
> Krzysztof Kozlowski came with the suggestion for Loongson to replace
> "skip-scan" with "status", during the review of their v1 patch.
> 
> In any case, that question will only recur one level lower - in U-Boot,
> where we make an effort to keep device trees in sync in Linux. Why would
> U-Boot need to do things to a disabled device? :)
> 
> > Maybe this should be a different DT property (not "status").  Then PCI
> > enumeration could work normally and 6fffbc7ae137 wouldn't be in the
> > way.
> 
> I'm not quite sure where you're going with this. More concretely?


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