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Date:   Wed, 14 Apr 2021 16:03:50 -0500
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Ingmar Klein <ingmar_klein@web.de>
Cc:     bhelgaas@google.com, linux-pci@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Alex Williamson <alex.williamson@redhat.com>
Subject: Re: QCA6174 pcie wifi: Add pci quirks
Message-ID: <20210414210350.GA2537653@bjorn-Precision-5520>
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[+cc Alex]

On Fri, Apr 09, 2021 at 11:26:33AM +0200, Ingmar Klein wrote:
> Edit: Retry, as I did not consider, that my mail-client would make this
> party html.
> 
> Dear maintainers,
> I recently encountered an issue on my Proxmox server system, that
> includes a Qualcomm QCA6174 m.2 PCIe wifi module.
> https://deviwiki.com/wiki/AIRETOS_AFX-QCA6174-NX
> 
> On system boot and subsequent virtual machine start (with passed-through
> QCA6174), the VM would just freeze/hang, at the point where the ath10k
> driver loads.
> Quick search in the proxmox related topics, brought me to the following
> discussion, which suggested a PCI quirk entry for the QCA6174 in the kernel:
> https://forum.proxmox.com/threads/pcie-passthrough-freezes-proxmox.27513/
> 
> I then went ahead, got the Proxmox kernel source (v5.4.106) and applied
> the attached patch.
> Effect was as hoped, that the VM hangs are now gone. System boots and
> runs as intended.
> 
> Judging by the existing quirk entries for Atheros, I would think, that
> my proposed "fix" could be included in the vanilla kernel.
> As far as I saw, there is no entry yet, even in the latest kernel sources.

This would need a signed-off-by; see
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?id=v5.11#n361

This is an old issue, and likely we'll end up just applying this as
yet another quirk.  But looking at c3e59ee4e766 ("PCI: Mark Atheros
AR93xx to avoid bus reset"), where it started, it seems to be
connected to 425c1b223dac ("PCI: Add Virtual Channel to save/restore
support").

I'd like to dig into that a bit more to see if there are any clues.
AFAIK Linux itself still doesn't use VC at all, and 425c1b223dac added
a fair bit of code.  I wonder if we're restoring something out of
order or making some simple mistake in the way to restore VC config.

> diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
> index 706f27a86a8e..ecfe80ec5b9c 100644
> --- a/drivers/pci/quirks.c
> +++ b/drivers/pci/quirks.c
> @@ -3584,6 +3584,7 @@ DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_ATHEROS, 0x0032, quirk_no_bus_reset);
>  DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_ATHEROS, 0x003c, quirk_no_bus_reset);
>  DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_ATHEROS, 0x0033, quirk_no_bus_reset);
>  DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_ATHEROS, 0x0034, quirk_no_bus_reset);
> +DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_ATHEROS, 0x003e, quirk_no_bus_reset);
>  
>  /*
>   * Root port on some Cavium CN8xxx chips do not successfully complete a bus