linux-kernel - Re: [PATCH v2] PCI/MSI: Don't touch MSI bits when the PCI device is disconnected

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Date:   Wed, 14 Nov 2018 13:23:33 -0700
From:   Keith Busch <keith.busch@...el.com>
To:     Alex_Gagniuc@...lteam.com
Cc:     helgaas@...nel.org, oohall@...il.com, gregkh@...uxfoundation.org,
        mr.nuke.me@...il.com, linux-pci@...r.kernel.org,
        Austin.Bolen@...l.com, Shyam.Iyer@...l.com,
        linux-kernel@...r.kernel.org, jonathan.derrick@...el.com,
        lukas@...ner.de, ruscur@...sell.cc, sbobroff@...ux.ibm.com,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2] PCI/MSI: Don't touch MSI bits when the PCI device is
 disconnected

On Wed, Nov 14, 2018 at 07:22:04PM +0000, Alex_Gagniuc@...lteam.com wrote:
> On 11/14/2018 12:00 AM, Bjorn Helgaas wrote:
> > Just to make sure we're on the same page, can you point me to this
> > rule?  I do see that OSPM must request control of AER using _OSC
> > before it touches the AER registers.  What I don't see is the
> > connection between firmware-first and the AER registers.
> 
> ACPI 6.2 - 6.2.11.3, Table 6-197:
> 
> PCI Express Advanced Error Reporting control:
>   * The firmware sets this bit to 1 to grant control over PCI Express 
> Advanced Error Reporting. If firmware allows the OS control of this 
> feature, then in the context of the _OSC method it must ensure that 
> error messages are routed to device interrupts as described in the PCI 
> Express Base Specification[...]
> 
> Now I'm confused too:
>   * HEST -> __aer_firmware_first
> 	This is used for touching/not touching AER bits
>   * _OSC -> bridge->native_aer
> 	Used to enable/not enable AER portdrv service
> Maybe Keith knows better why we're doing it this way. From ACPI text, it 
> doesn't seem that control of AER would be tied to HEST entries, although 
> in practice, it is.

I'm not sure, that predates me.  HEST does have a FIRMWARE_FIRST flag, but
spec does not say anymore on relation to _OSC control or AER capability.
Nothing in PCIe spec either.

I also don't know why Linux disables the AER driver if only one
device has a FIRMWARE_FIRST HEST. Shouldn't that just be a per-device
decision?

> > The closest I can find is the "Enabled" field in the HEST PCIe
> > AER structures (ACPI v6.2, sec 18.3.2.4, .5, .6), where it says:
> > 
> >    If the field value is 1, indicates this error source is
> >    to be enabled.
> > 
> >    If the field value is 0, indicates that the error source
> >    is not to be enabled.
> > 
> >    If FIRMWARE_FIRST is set in the flags field, the Enabled
> >    field is ignored by the OSPM.
> > 
> > AFAICT, Linux completely ignores the Enabled field in these
> > structures.
> 
> I don't think ignoring the field is a problem:
>   * With FFS, OS should ignore it.
>   * Without FFS, we have control, and we get to make the decisions anyway.
> In the latter case we decide whether to use AER, independent of the crap 
> in ACPI. I'm not even sure why "Enabled" matters in native AER handling. 
> Probably one of the check-boxes in "Binary table designer's handbook"?

And why doesn't Linux do anything with _OSC response other than logging
it? If OS control wasn't granted, shouldn't that take priority over HEST?