Date:   Fri, 22 Oct 2021 19:17:28 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Jeremy Linton <jeremy.linton@....com>,
        Bjorn Helgaas <helgaas@...nel.org>, linux-pci@...r.kernel.org,
        lorenzo.pieralisi@....com, nsaenz@...nel.org, bhelgaas@...gle.com,
        rjw@...ysocki.net, lenb@...nel.org, robh@...nel.org, kw@...ux.com,
        bcm-kernel-feedback-list@...adcom.com, linux-acpi@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-rpi-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/4] PCI: brcmstb: Add ACPI config space quirk

On Friday 22 October 2021 10:04:36 Florian Fainelli wrote:
> On 10/5/21 7:07 PM, Florian Fainelli wrote:
> > 
> > 
> > On 10/5/2021 3:25 PM, Jeremy Linton wrote:
> >> Hi,
> >>
> >> On 10/5/21 2:43 PM, Pali Rohár wrote:
> >>> Hello!
> >>>
> >>> On Tuesday 05 October 2021 10:57:18 Jeremy Linton wrote:
> >>>> Hi,
> >>>>
> >>>> On 10/5/21 10:32 AM, Bjorn Helgaas wrote:
> >>>>> On Thu, Aug 26, 2021 at 02:15:55AM -0500, Jeremy Linton wrote:
> >>>>>> Additionally, some basic bus/device filtering exist to avoid sending
> >>>>>> config transactions to invalid devices on the RP's primary or
> >>>>>> secondary bus. A basic link check is also made to assure that
> >>>>>> something is operational on the secondary side before probing the
> >>>>>> remainder of the config space. If either of these constraints are
> >>>>>> violated and a config operation is lost in the ether because an EP
> >>>>>> doesn't respond an unrecoverable SERROR is raised.
> >>>>>
> >>>>> It's not "lost"; I assume the root port raises an error because it
> >>>>> can't send a transaction over a link that is down.
> >>>>
> >>>> The problem is AFAIK because the root port doesn't do that.
> >>>
> >>> Interesting! Does it mean that PCIe Root Complex / Host Bridge (which I
> >>> guess contains also logic for Root Port) does not signal transaction
> >>> failure for config requests? Or it is just your opinion? Because I'm
> >>> dealing with similar issues and I'm trying to find a way how to detect
> >>> if some PCIe IP signal transaction error via AXI SLVERR response OR it
> >>> just does not send any response back. So if you know some way how to
> >>> check which one it is, I would like to know it too.
> >>
> >> This is my _opinion_ based on what I've heard of some other IP
> >> integration issues, and what i've seen poking at this one from the
> >> perspective of a SW guy rather than a HW guy. So, basically worthless.
> >> But, you should consider that most of these cores/interconnects aren't
> >> aware of PCIe completion semantics so its the root ports
> >> responsibility to say, gracefully translate a non-posted write that
> >> doesn't have a completion for the interconnects its attached to,
> >> rather than tripping something generic like a SLVERR.
> >>
> >> Anyway, for this I would poke around the pile of exception registers,
> >> with your specific processors manual handy because a lot of them are
> >> implementation defined.
> > 
> > I should be able to get you an answer in the new few days whether
> > configuration space requests also generate an error towards the ARM CPU,
> > since memory space requests most definitively do.
> 
> Did not get an answer from the design team, but going through our bug
> tracker, there were evidences of configuration space accesses also
> generating external aborts:
> 
> [    8.988237] Unhandled fault: synchronous external abort (0x96000210) at 0xffffff8009539004
> [    9.026698] PC is at pci_generic_config_read32+0x30/0xb0

So this is error caused by reading from config space.

Can you check if also writing to config space can trigger some crash? If
yes, I would like to know if write would be also synchronous or rather
asynchronous abort.

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