Re: [PATCH] pci: Don't set RCB bit in LNKCTL if the upstream bridge hasn't

[Bjorn Helgaas]

On Thu, Oct 27, 2016 at 07:42:27AM +0200, Hannes Reinecke wrote:
> On 10/26/2016 09:43 PM, Bjorn Helgaas wrote:
> > Hi Johannes,
> > 
> > On Wed, Oct 26, 2016 at 03:53:34PM +0200, Johannes Thumshirn wrote:
> >> The Read Completion Boundary bit must only be set on a device or endpoint if
> >> it is set on the upstream bridge.
> >>
> >> Fixes: 7a1562d4f ("PCI: Apply _HPX Link Control settings to all devices with a link")
> > 
> > Can you please include a spec citation and a pointer to the bug report?
> > 
> PCI Express Base Specification 1.1,
> section 2.3.1.1. Data Return for Read Requests:
> 
> The Read Completion Boundary (RCB) parameter determines the naturally
> aligned address boundaries on which a Read Request may be serviced with
> multiple Completions
> o For a Root Complex, RCB is 64 bytes or 128 bytes
>   o This value is reported through a configuration register
>     (see Section 7.8)
>   Note: Bridges and Endpoints may implement a corresponding command
>   bit which may be set by system software to indicate the RCB value
>   for the Root Complex, allowing the Bridge/Endpoint to optimize its
>   behavior when the Root Complexâs RCB is 128 bytes.
> o For all other system elements, RCB is 128 bytes
> 
> In this particular case the _HPX method was causing the RCB for all PCI
> devices to be set to 128 bytes, while the root bridge remained at 64 bytes.
> While this is arguably a BIOS bug, earlier linux version (ie without the
> mentioned patch) were running fine, so this is actually a regression.

Thanks!  I can fold this into the changelog.

I assume you didn't mention a bugzilla or similar URL because this was
found internally?  I'd still like a clue about what this issue looks
like to a user, because that helps connect future problem reports with
this fix.

And I suppose that since 7a1562d4f2d0 appeared in v3.18, we maybe
should consider marking the fix for stable?

Bjorn