Date:   Fri, 16 Oct 2020 12:59:29 +0100
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Zhiqiang Hou <Zhiqiang.Hou@....com>, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, robh@...nel.org, bhelgaas@...gle.com,
        gustavo.pimentel@...opsys.com
Subject: Re: [PATCH] PCI: dwc: Added link up check in map_bus of
 dw_child_pcie_ops

On Thu, Oct 15, 2020 at 05:47:38PM -0500, Bjorn Helgaas wrote:
> On Wed, Sep 16, 2020 at 01:41:30PM +0800, Zhiqiang Hou wrote:
> > From: Hou Zhiqiang <Zhiqiang.Hou@....com>
> > 
> > On NXP Layerscape platforms, it results in SError in the
> > enumeration of the PCIe controller, which is not connecting
> > with an Endpoint device. And it doesn't make sense to
> > enumerate the Endpoints when the PCIe link is down. So this
> > patch added the link up check to avoid to fire configuration
> > transactions on link down bus.
> 
> Lorenzo already applied this, but a couple questions:

Happy to drop it - I merged it because it is a regression but
that's certainly not a proper fix either.

> You call out NXP Layerscape specifically, but doesn't this affect
> other DWC-based platforms, too?  You later mentioned imx6, Kishon
> mentioned dra7xx, Michael mentioned ls1028a, Naresh mentioned ls2088
> (probably both the same as your "NXP Layerscape").
> 
> The backtrace below contains a bunch of irrelevant info.  The
> timestamps are pointless.  The backtrace past
> pci_scan_single_device+0x80/0x100 or so really doesn't add anything
> either.

I can trim the log message easily but I certainly agree with you
the problem in this patch is the usual racy link check that still
nobody can explains and that we were trying to remove.

It would be very good if the respective platform maintainers went
the extra mile to help here, Rob and I don't have this HW.

Shall we drop this patch and investigate further (possibly hitting
-rc1 with a fix containing a decent explanation ?)

> 
> It'd be nice to have a comment in the code because the code *looks*
> wrong and racy.  Without a hint, everybody who sees it will have to
> dig through the history to see why we tolerate the race.

+1, see above, ready to drop it.

Lorenzo

> > [    0.807773] SError Interrupt on CPU2, code 0xbf000002 -- SError
> > [    0.807775] CPU: 2 PID: 1 Comm: swapper/0 Not tainted 5.9.0-rc5-next-20200914-00001-gf965d3ec86fa #67
> > [    0.807776] Hardware name: LS1046A RDB Board (DT)
> > [    0.807777] pstate: 20000085 (nzCv daIf -PAN -UAO BTYPE=--)
> > [    0.807778] pc : pci_generic_config_read+0x3c/0xe0
> > [    0.807778] lr : pci_generic_config_read+0x24/0xe0
> > [    0.807779] sp : ffff80001003b7b0
> > [    0.807780] x29: ffff80001003b7b0 x28: ffff80001003ba74
> > [    0.807782] x27: ffff000971d96800 x26: ffff00096e77e0a8
> > [    0.807784] x25: ffff80001003b874 x24: ffff80001003b924
> > [    0.807786] x23: 0000000000000004 x22: 0000000000000000
> > [    0.807788] x21: 0000000000000000 x20: ffff80001003b874
> > [    0.807790] x19: 0000000000000004 x18: ffffffffffffffff
> > [    0.807791] x17: 00000000000000c0 x16: fffffe0025981840
> > [    0.807793] x15: ffffb94c75b69948 x14: 62203a383634203a
> > [    0.807795] x13: 666e6f635f726568 x12: 202c31203d207265
> > [    0.807797] x11: 626d756e3e2d7375 x10: 656877202c307830
> > [    0.807799] x9 : 203d206e66766564 x8 : 0000000000000908
> > [    0.807801] x7 : 0000000000000908 x6 : ffff800010900000
> > [    0.807802] x5 : ffff00096e77e080 x4 : 0000000000000000
> > [    0.807804] x3 : 0000000000000003 x2 : 84fa3440ff7e7000
> > [    0.807806] x1 : 0000000000000000 x0 : ffff800010034000
> > [    0.807808] Kernel panic - not syncing: Asynchronous SError Interrupt
> > [    0.807809] CPU: 2 PID: 1 Comm: swapper/0 Not tainted 5.9.0-rc5-next-20200914-00001-gf965d3ec86fa #67
> > [    0.807810] Hardware name: LS1046A RDB Board (DT)
> > [    0.807811] Call trace:
> > [    0.807812]  dump_backtrace+0x0/0x1c0
> > [    0.807813]  show_stack+0x18/0x28
> > [    0.807814]  dump_stack+0xd8/0x134
> > [    0.807814]  panic+0x180/0x398
> > [    0.807815]  add_taint+0x0/0xb0
> > [    0.807816]  arm64_serror_panic+0x78/0x88
> > [    0.807817]  do_serror+0x68/0x180
> > [    0.807818]  el1_error+0x84/0x100
> > [    0.807818]  pci_generic_config_read+0x3c/0xe0
> > [    0.807819]  dw_pcie_rd_other_conf+0x78/0x110
> > [    0.807820]  pci_bus_read_config_dword+0x88/0xe8
> > [    0.807821]  pci_bus_generic_read_dev_vendor_id+0x30/0x1b0
> > [    0.807822]  pci_bus_read_dev_vendor_id+0x4c/0x78
> > [    0.807823]  pci_scan_single_device+0x80/0x100
> > [    0.807824]  pci_scan_slot+0x38/0x130
> > [    0.807825]  pci_scan_child_bus_extend+0x54/0x2a0
> > [    0.807826]  pci_scan_child_bus+0x14/0x20
> > [    0.807827]  pci_scan_bridge_extend+0x230/0x570
> > [    0.807828]  pci_scan_child_bus_extend+0x134/0x2a0
> > [    0.807829]  pci_scan_root_bus_bridge+0x64/0xf0
> > [    0.807829]  pci_host_probe+0x18/0xc8
> > [    0.807830]  dw_pcie_host_init+0x220/0x378
> > [    0.807831]  ls_pcie_probe+0x104/0x140
> > [    0.807832]  platform_drv_probe+0x54/0xa8
> > [    0.807833]  really_probe+0x118/0x3e0
> > [    0.807834]  driver_probe_device+0x5c/0xc0
> > [    0.807835]  device_driver_attach+0x74/0x80
> > [    0.807835]  __driver_attach+0x8c/0xd8
> > [    0.807836]  bus_for_each_dev+0x7c/0xd8
> > [    0.807837]  driver_attach+0x24/0x30
> > [    0.807838]  bus_add_driver+0x154/0x200
> > [    0.807839]  driver_register+0x64/0x120
> > [    0.807839]  __platform_driver_probe+0x7c/0x148
> > [    0.807840]  ls_pcie_driver_init+0x24/0x30
> > [    0.807841]  do_one_initcall+0x60/0x1d8
> > [    0.807842]  kernel_init_freeable+0x1f4/0x24c
> > [    0.807843]  kernel_init+0x14/0x118
> > [    0.807843]  ret_from_fork+0x10/0x34
> > [    0.807854] SMP: stopping secondary CPUs
> > [    0.807855] Kernel Offset: 0x394c64080000 from 0xffff800010000000
> > [    0.807856] PHYS_OFFSET: 0xffff8bfd40000000
> > [    0.807856] CPU features: 0x0240022,21806000
> > [    0.807857] Memory Limit: none
> > 
> > Fixes: c2b0c098fbd1 ("PCI: dwc: Use generic config accessors")
> > Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@....com>
> > ---
> >  drivers/pci/controller/dwc/pcie-designware-host.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
> > index c01c9d2fb3f9..e82b518430c5 100644
> > --- a/drivers/pci/controller/dwc/pcie-designware-host.c
> > +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
> > @@ -442,6 +442,9 @@ static void __iomem *dw_pcie_other_conf_map_bus(struct pci_bus *bus,
> >  	struct pcie_port *pp = bus->sysdata;
> >  	struct dw_pcie *pci = to_dw_pcie_from_pp(pp);
> >  
> > +	if (!dw_pcie_link_up(pci))
> > +		return NULL;
> > +
> >  	busdev = PCIE_ATU_BUS(bus->number) | PCIE_ATU_DEV(PCI_SLOT(devfn)) |
> >  		 PCIE_ATU_FUNC(PCI_FUNC(devfn));
> >  
> > -- 
> > 2.17.1
> > 

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