RE: [PATCH] PCI: dwc: Added link up check in map_bus of dw_child_pcie_ops

[Z.q. Hou]

Hi Lorenzo,

Thanks a lot for your comments!

> -----Original Message-----
> From: Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx>
> Sent: 2020年9月28日 17:39
> To: Z.q. Hou <zhiqiang.hou@xxxxxxx>
> Cc: Rob Herring <robh@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; PCI
> <linux-pci@xxxxxxxxxxxxxxx>; Bjorn Helgaas <bhelgaas@xxxxxxxxxx>;
> Gustavo Pimentel <gustavo.pimentel@xxxxxxxxxxxx>; Michael Walle
> <michael@xxxxxxxx>; Ard Biesheuvel <ardb@xxxxxxxxxx>
> Subject: Re: [PATCH] PCI: dwc: Added link up check in map_bus of
> dw_child_pcie_ops
> 
> On Thu, Sep 24, 2020 at 04:24:47AM +0000, Z.q. Hou wrote:
> > Hi Rob,
> >
> > Thanks a lot for your comments!
> >
> > > -----Original Message-----
> > > From: Rob Herring <robh@xxxxxxxxxx>
> > > Sent: 2020年9月18日 23:28
> > > To: Z.q. Hou <zhiqiang.hou@xxxxxxx>
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx; PCI <linux-pci@xxxxxxxxxxxxxxx>;
> > > Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx>; Bjorn Helgaas
> > > <bhelgaas@xxxxxxxxxx>; Gustavo Pimentel
> > > <gustavo.pimentel@xxxxxxxxxxxx>; Michael Walle
> <michael@xxxxxxxx>;
> > > Ard Biesheuvel <ardb@xxxxxxxxxx>
> > > Subject: Re: [PATCH] PCI: dwc: Added link up check in map_bus of
> > > dw_child_pcie_ops
> > >
> > > On Fri, Sep 18, 2020 at 5:02 AM Z.q. Hou <zhiqiang.hou@xxxxxxx>
> wrote:
> > > >
> > > > Hi Rob,
> > > >
> > > > Thanks a lot for your comments!
> > > >
> > > > > -----Original Message-----
> > > > > From: Rob Herring <robh@xxxxxxxxxx>
> > > > > Sent: 2020年9月17日 4:29
> > > > > To: Z.q. Hou <zhiqiang.hou@xxxxxxx>
> > > > > Cc: linux-kernel@xxxxxxxxxxxxxxx; PCI
> > > > > <linux-pci@xxxxxxxxxxxxxxx>; Lorenzo Pieralisi
> > > > > <lorenzo.pieralisi@xxxxxxx>; Bjorn Helgaas
> > > > > <bhelgaas@xxxxxxxxxx>; Gustavo Pimentel
> > > > > <gustavo.pimentel@xxxxxxxxxxxx>; Michael Walle
> > > <michael@xxxxxxxx>;
> > > > > Ard Biesheuvel <ardb@xxxxxxxxxx>
> > > > > Subject: Re: [PATCH] PCI: dwc: Added link up check in map_bus of
> > > > > dw_child_pcie_ops
> > > > >
> > > > > On Tue, Sep 15, 2020 at 11:49 PM Zhiqiang Hou
> > > <Zhiqiang.Hou@xxxxxxx>
> > > > > wrote:
> > > > > >
> > > > > > From: Hou Zhiqiang <Zhiqiang.Hou@xxxxxxx>
> > > > > >
> > > > > > 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.
> > > > >
> > > > > Michael reported the same issue as well.
> > > > >
> > > > > What happens if the link goes down between the check and the
> access?
> > > >
> > > > This patch cannot cover this case, and will get the SError.
> > > > But I think it makes sense to avoid firing transactions on link down bus.
> > >
> > > That's impossible to do without a race even in h/w.
> >
> > Agree.
> >
> > >
> > > > > It's a racy check. I'd like to find an alternative solution.
> > > > > It's even worse if Layerscape is used in ECAM mode. I looked at
> > > > > the EDK2 setup for layerscape[1] and it looks like root ports
> > > > > are just skipped if link
> > > is down.
> > > > > Maybe a link down just never happens once up, but if so, then we
> > > > > only need to check it once and fail probe.
> > > >
> > > > Many customers connect the FPGA Endpoint, which may establish PCIe
> > > > link after the PCIe enumeration and then rescan the PCIe bus, so I
> > > > think it should not exit the probe of root port even if there is
> > > > not link up
> > > during enumeration.
> > >
> > > That's a good reason. I want to unify the behavior here as it varies
> > > per platform currently and wasn't sure which way to go.
> > >
> > >
> > > > > I've dug into this a bit more and am curious about the
> > > > > PCIE_ABSERR register setting which is set to:
> > > > >
> > > > > #define PCIE_ABSERR_SETTING 0x9401 /* Forward error of
> > > > > non-posted request */
> > > > >
> > > > > It seems to me this is not what we want at least for config
> > > > > accesses, but commit 84d897d6993 where this was added seems to
> > > > > say otherwise. Is it not possible to configure the response per access
> type?
> > > >
> > > > Thanks a lot for your investigation!
> > > > The story is like this: Some customers worry about these silent
> > > > error (DWC PCIe IP won't forward the error of outbound non-post
> > > > request by default), so we were pushed to enable the error
> > > > forwarding to AXI in the commit
> > > > 84d897d6993 as you saw. But it cannot differentiate the config
> > > > transactions from the MEM_rd, except the Vendor ID access, which
> > > > is controlled by a separate bit and it was set to not forward
> > > > error of access
> > > of Vendor ID.
> > > > So we think it's okay to enable the error forwarding, the SError
> > > > should not occur, because after the enumeration it won't access
> > > > the
> > > non-existent functions.
> > >
> > > We've rejected upstream support for platforms aborting on config
> > > accesses[1]. I think there's clear consensus that aborting is the
> > > wrong behavior.
> > >
> > > Do MEM_wr errors get forwarded? Seems like that would be enough.
> > > Also, wouldn't page faults catch most OOB accesses anyways? You need
> > > things page aligned anyways with an IOMMU and doing userspace access
> > > or guest assignment.
> >
> > Yes, errors of MEM_wr can be forwarded.
> >
> > >
> > > Here's another idea, how about only enabling forwarding errors if
> > > the link is up? If really would need to be configured any time the
> > > link state changes rather than just at probe. I'm not sure if you
> > > have a way to disable it on link down though.
> >
> > Dug deeper into this issue and found the setting of not forwarding
> > error of non-existent Vender ID access counts on the link partner: 1.
> > When there is a link partner (namely link up), it will return 0xffff
> > when read non-existent function Vendor ID and won't forward error to
> > AXI.  2. When no link partner (link down), it will forward the error
> > of reading non-existent function Vendor ID to AXI and result in
> > SError.
> >
> > I think this is a DWC PCIe IP specific issue but not get feedback from
> > design team.  I'm thinking to disable this error forwarding just like
> > other platforms, since when these errors (UR, CA and CT) are detected,
> > AER driver can also report the error and try to recover.
> 
> I take this as you shall send a patch to fix this issue shortly, is this correct ?

The issue becomes complex:
I reviewed the DWC PCIe databook of verion 4.40a which is used on Layerscape platforms, and it said that " Your RC application should not generate CFG requests until it has confirmed that the link is up by sampling the smlh_link_up and rmlh_link_up outputs".
So, the link up checking should not be remove before each outbound CFG access.
Gustavo, can you share more details on the link up checking? Does it only exist in the 4.40a?

For the feature of error forwarding to AXI that enabled on Layerscape PCIe, I will send a patch to disable it today.

Thanks,
Zhiqiang 

> 
> Thanks,
> Lorenzo