Re: [PATCH] PCI: dwc: Fix interrupt race in when handling MSI

[Trent Piepho <tpiepho@impinj.com>]

On Thu, 2018-11-08 at 09:49 +0000, Marc Zyngier wrote:
> On 07/11/18 20:17, Trent Piepho wrote:
> > On Wed, 2018-11-07 at 18:41 +0000, Marc Zyngier wrote:
> > > On 06/11/18 19:40, Trent Piepho wrote:
> > > > 
> > > > What about stable kernels that don't have the hierarchical API?
> > > 
> > > My goal is to fix mainline first. Once we have something that works on
> > > mainline, we can look at propagating the fix to other versions. But
> > > mainline always comes first.
> > 
> > This is a regression that went into 4.14.  Wouldn't the appropriate
> > action for the stable series be to undo the regression?
> 
> This is not how stable works. Stable kernels *only* contain patches that
> are backported from mainline, and do not take standalone patch.
> 
> Furthermore, your fix is to actually undo someone else's fix. Who is
> right? In the absence of any documentation, the answer is "nobody".

Little more history to this bug.  The code was originally the way it is
now, but this same bug was fixed in 2013 in https://patchwork.kernel.or
g/patch/3333681/

Then that lasted four years until it was changed Aug 2017 in https://pa
tchwork.kernel.org/patch/9893303/

That lasted just six months until someone tried to revert it, https://p
atchwork.kernel.org/patch/9893303/

Seems pretty clear the way it is now is much worse than the way it was
before, even if the previous design may have had another flaw.  Though
I've yet to see anyone point out something makes the previous design
broken.  Sub-optimal yes, but not broken.

> Anything can be backported to stable once we understand the issue. At
> the moment, we're just playing games moving stuff around and hope
> nothing else will break. That's not a sustainable way of maintaining
> this driver. At the moment, the only patch I'm inclined to propose until
> we get an actual interrupt handling flow from Synopsys is to mark this
> driver as "BROKEN".

It feels like you're using this bug to hold designware hostage in a
broken kernel, and me along with them.  I don't have the documentation,
no one does, there's no way for me to give you want you want.  But I've
got hardware that doesn't work in the mainline kernel.