Re: [PATCH v1] drivers: pci: introduce configurable delay for Rockchip PCIe bus scan

[Peter Geis <pgwipeout@gmail.com>]

On Mon, May 15, 2023 at 12:51 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> On Sat, May 13, 2023 at 07:40:12AM -0400, Peter Geis wrote:
> > On Fri, May 12, 2023 at 9:24 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > >
> > > [+cc ARM64 folks, in case you have abort handling tips; thread at:
> > > https://lore.kernel.org/r/20230509153912.515218-1-vincenzopalazzodev@gmail.com]
> > >
> > > Pine64 RockPro64 panics while enumerating some PCIe devices.  Adding a
> > > delay avoids the panic.  My theory is a PCIe Request Retry Status to a
> > > Vendor ID config read causes an abort that we don't handle.
> > >
> > > > On Tue, May 09, 2023 at 05:39:12PM +0200, Vincenzo Palazzo wrote:
> > > >> ...
> > > >> [    1.229856] SError Interrupt on CPU4, code 0xbf000002 -- SError
> > > >> [    1.229860] CPU: 4 PID: 1 Comm: swapper/0 Not tainted 5.9.9-2.0-MANJARO-ARM
> > > >> #1
> > > >> [    1.229862] Hardware name: Pine64 RockPro64 v2.1 (DT)
> > > >> [    1.229864] pstate: 60000085 (nZCv daIf -PAN -UAO BTYPE=--)
> > > >> [    1.229866] pc : rockchip_pcie_rd_conf+0xb4/0x270
> > > >> [    1.229868] lr : rockchip_pcie_rd_conf+0x1b4/0x270
> > > >> ...
> > > >> [    1.229939] Kernel panic - not syncing: Asynchronous SError Interrupt
> > > >> ...
> > > >> [    1.229955]  nmi_panic+0x8c/0x90
> > > >> [    1.229956]  arm64_serror_panic+0x78/0x84
> > > >> [    1.229958]  do_serror+0x15c/0x160
> > > >> [    1.229960]  el1_error+0x84/0x100
> > > >> [    1.229962]  rockchip_pcie_rd_conf+0xb4/0x270
> > > >> [    1.229964]  pci_bus_read_config_dword+0x6c/0xd0
> > > >> [    1.229966]  pci_bus_generic_read_dev_vendor_id+0x34/0x1b0
> > > >> [    1.229968]  pci_scan_single_device+0xa4/0x144
> > >
> > > On Fri, May 12, 2023 at 12:46:21PM +0200, Vincenzo Palazzo wrote:
> > > > ... Is there any way to tell the kernel "hey we need some more time
> > > > here"?
> > >
> > > We enumerate PCI devices by trying to read the Vendor ID of every
> > > possible device address (see pci_scan_slot()).  On PCIe, if a device
> > > doesn't exist at that address, the Vendor ID config read will be
> > > terminated with Unsupported Request (UR) status.  This is normal
> > > and happens every time we enumerate devices.
> > >
> > > The crash doesn't happen every time we enumerate, so I don't think
> > > this UR is the problem.  Also, if it *were* the problem, adding a
> > > delay would not make any difference.
> >
> > Is this behavior different if there is a switch device forwarding on
> > the UR? On rk3399 switches are completely non-functional because of
> > the panic, which is observed in the output of the dmesg in [2] with
> > the hack patch enabled. Considering what you just described it looks
> > like the forwarded UR for each non-existent device behind the switch
> > is causing an serror.
>
> I don't know exactly what the panic looks like, but I wouldn't expect
> UR handling to be different when there's a switch.
>
> pcie-rockchip-host.c does handle devices on the root bus (00)
> differently than others because rockchip_pcie_valid_device() knows
> that device 00:00 is the only device on the root bus.  That part makes
> sense because 00:00 is built into the SoC.
>
> I'm a little suspicious of the fact that rockchip_pcie_valid_device()
> also enforces that bus 01 can only have a single device on it.  No
> other *_pcie_valid_device() implementations enforce that.  It's true
> that traditional PCIe devices can only implement device 00, but ARI
> relaxes that by reusing the Device Number as extended Function Number
> bits.

Bjorn, great catch, thank you!

I suspect you're actually onto the core of the problem. Looking
through various other drivers that implement _pcie_valid_device they
all appear to file similar restrictions on scanning for devices. The
drivers are all similar enough that I am starting to suspect they are
all running some version of the same bugged IP. Then I came across
advk_pcie_pio_is_running() in pci-aardvark.c, which describes our
issue pretty spot on including the same exact SError. Interestingly
enough they made a TF-A patch [3] to catch and handle the error
without ever passing it to the kernel. Other limitations they added
are ensuring reads are not attempted while the link is down.
pci-aardvark.c also implements limitations on Completion Retry Status.
It has given me ideas for solving the problem.

Very Respectfully,
Peter Geis

[3] https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/commit/?id=3c7dcdac5c50

>
> > > There *is* a way for a PCIe device to say "I need more time".  It does
> > > this by responding to that Vendor ID config read with Request Retry
> > > Status (RRS, aka CRS in older specs), which means "I'm not ready yet,
> > > but I will be ready in the future."  Adding a delay would definitely
> > > make a difference here, so my guess is this is what's happening.
> > >
> > > Most root complexes return ~0 data to the CPU when a config read
> > > terminates with UR or RRS.  It sounds like rockchip does this for UR
> > > but possibly not for RRS.
> > >
> > > There is a "RRS Software Visibility" feature, which is supposed to
> > > turn the RRS into a special value (Vendor ID == 0x0001), but per [1],
> > > rockchip doesn't support it (lspci calls it "CRSVisible").
> > >
> > > But the CPU load instruction corresponding to the config read has to
> > > complete by reading *something* or else be aborted.  It sounds like
> > > it's aborted in this case.  I don't know the arm64 details, but if we
> > > could catch that abort and determine that it was an RRS and not a UR,
> > > maybe we could fabricate the magic RRS 0x0001 value.
> > >
> > > imx6q_pcie_abort_handler() does something like that, although I think
> > > it's for arm32, not arm64.  But obviously we already catch the abort
> > > enough to dump the register state and panic, so maybe there's a way to
> > > extend that?
> >
> > Perhaps a hook mechanism that allows drivers to register with the
> > serror handler and offer to handle specific errors before the generic
> > code causes the system panic?
> >
> > Very Respectfully,
> > Peter Geis
> >
> > [2] https://lore.kernel.org/linux-pci/CAMdYzYqn3L7x-vc+_K6jG0EVTiPGbz8pQ-N1Q1mRbcVXE822Yg@mail.gmail.com/
> >
> > >
> > > Bjorn
> > >
> > > [1] https://lore.kernel.org/linux-pci/CAMdYzYpOFAVq30N+O2gOxXiRtpoHpakFg3LKq3TEZq4S6Y0y0g@mail.gmail.com/
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