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

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Message-ID: <ZHZ4TFjFLrKeHPGi@bhelgaas>
Date:   Tue, 30 May 2023 17:27:24 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     linux-pci@...r.kernel.org, netdev@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Rob Herring <robh@...nel.org>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Michael Walle <michael@...le.cc>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH pci] PCI: don't skip probing entire device if first fn OF
 node has status = "disabled"

On Wed, May 31, 2023 at 01:04:36AM +0300, Vladimir Oltean wrote:
> On Tue, May 30, 2023 at 04:58:55PM -0500, Bjorn Helgaas wrote:
> > Can you write this description in terms of PCI topology?  The
> > nitty-gritty SERDES details are not relevant at this level, except to
> > say that Function 0 is present in some cases but not others, and when
> > it is not present, *other* functions may be present.
> 
> No. It is to say that within the device, all PCIe functions (including 0)
> are always available and have the same number, but depending on SERDES
> configuration, their PCIe presence might be practically useful or not.
> So that's how function 0 may end having status = "disabled" in the
> device tree.
>
> > Sigh.  Per spec (PCIe r6.0, sec 7.5.1.1.9), software is not permitted
> > to probe for Functions other than 0 unless "explicitly indicated by
> > another mechanism, such as an ARI or SR-IOV Capability."
> > 
> > Does it "work" to probe when the spec prohibits it?  Probably.  Does
> > it lead to some breakage elsewhere eventually?  Quite possibly.  They
> > didn't put "software must not probe" in the spec just to make
> > enumeration faster.
> > 
> > So I'm a little grumpy about further complicating this already messy
> > path just to accommodate a new non-compliant SoC.  Everybody pays the
> > price of understanding all this stuff, and it doesn't seem in balance.
> > 
> > Can you take advantage of some existing mechanism like
> > PCI_SCAN_ALL_PCIE_DEVS or hypervisor_isolated_pci_functions() (which
> > could be renamed and made more general)?
> 
> Not responding yet to the rest of the email since it's not clear to me
> that you've understood function 0 is absolutely present and responds
> to all config space accesses - it's just disabled in the device tree
> because the user doesn't have something useful to do with it.

Ah, you're right, sorry I missed that.  Dispensing with the SERDES
details would make this more obvious.

Not sure why this needs to change the pci_scan_slot() path, since
Function 0 is present and enumerable even though it's not useful in
some cases.  Seems like something in pci_set_of_node() or a quirk
could do whatever you need to do.

Bjorn