Date:   Thu, 9 Mar 2023 11:38:36 -0600
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Shawn Guo <shawn.guo@linaro.org>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Maximilian Luz <luzmaximilian@gmail.com>,
        Lorenzo Pieralisi <lpieralisi@kernel.org>,
        Bjorn Andersson <quic_bjorande@quicinc.com>,
        linux-arm-msm@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-acpi@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] arm64: PCI: Add quirk for platforms running Windows
Message-ID: <20230309173836.GA1148798@bhelgaas>
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On Thu, Mar 09, 2023 at 10:52:13AM +0800, Shawn Guo wrote:
> + linux-arm-msm and MSM maintainer Bjorn
> 
> On Wed, Mar 08, 2023 at 12:53:10PM -0600, Bjorn Helgaas wrote:
> > On Mon, Feb 27, 2023 at 10:12:21AM +0800, Shawn Guo wrote:
> > > Commit 8fd4391ee717 ("arm64: PCI: Exclude ACPI "consumer" resources from
> > > host bridge windows") introduced a check to remove host bridge register
> > > resources for all arm64 platforms, with the assumption that the PNP0A03
> > > _CRS resources would always be host bridge registers and never as windows
> > > on arm64.
> > > 
> > > The assumption stands true until Qualcomm Snapdragon Windows laptops
> > > emerge.  These laptops describe host bridge windows in PNP0A03 _CRS
> > > resources instead.  For example, the Microsoft Surface Pro X has host
> > > bridges defined as
> > > 
> > >     Name (_HID, EisaId ("PNP0A08") /* PCI Express Bus */)  // _HID: Hardware ID
> > >     Name (_CID, EisaId ("PNP0A03") /* PCI Bus */)  // _CID: Compatible ID
> > > 
> > >     Method (_CRS, 0, NotSerialized)  // _CRS: Current Resource Settings
> > >     {
> > >         Name (RBUF, ResourceTemplate ()
> > >         {
> > >             Memory32Fixed (ReadWrite,
> > >                 0x60200000,         // Address Base
> > >                 0x01DF0000,         // Address Length
> > >                 )
> > >             WordBusNumber (ResourceProducer, MinFixed, MaxFixed, PosDecode,
> > >                 0x0000,             // Granularity
> > >                 0x0000,             // Range Minimum
> > >                 0x0001,             // Range Maximum
> > >                 0x0000,             // Translation Offset
> > >                 0x0002,             // Length
> > >                 ,, )
> > >         })
> > >         Return (RBUF) /* \_SB_.PCI0._CRS.RBUF */
> > >     }
> > > 
> > > The Memory32Fixed holds a host bridge window, but it's not properly
> > > defined as a "producer" resource.  Consequently the resource gets
> > > removed by kernel, and the BAR allocation fails later on:
> > > 
> > >     [ 0.150731] pci 0002:00:00.0: BAR 14: no space for [mem size 0x00100000]
> > >     [ 0.150744] pci 0002:00:00.0: BAR 14: failed to assign [mem size 0x00100000]
> > >     [ 0.150758] pci 0002:01:00.0: BAR 0: no space for [mem size 0x00004000 64bit]
> > >     [ 0.150769] pci 0002:01:00.0: BAR 0: failed to assign [mem size 0x00004000 64bit]
> > > 
> > > This eventually prevents the PCIe NVME drive from being accessible.
> > > 
> > > Add a quirk for these platforms to avoid the resource being removed.
> > > 
> > > Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
> > > ---
> > > We are running into the issue on more devices than just Surface Pro X
> > > now, so trying to sort it out with a quirk as suggested by Lorenzo [1].
> > 
> > One thing I don't like about this application of quirks is that the
> > list of affected platforms is likely to grow, which is an ongoing
> > burden for users and developers.
> 
> It's a very reasonable concern.  I really hope that Qualcomm will start
> thinking about Linux support on these machines in the future not too far
> away, so that the list will not grow too long.
> 
> > Can we have a conversation with Qualcomm about how they *intend* this
> > to work?  Linux is probably doing something wrong (interpreting
> > something differently than Windows does), and if we could fix that, we
> > have a better chance of future platforms working without quirks.
> 
> Today Qualcomm only ships and cares about Windows on these machines, but
> I believe it will change sooner or later.

I don't maintain arch/arm64/kernel/pci.c, but my opinion is that we
should not pursue quirks like this until we've tried really hard to
figure out a generic approach.

Bjorn