Re: [Qemu-devel] [PATCH] fsl-imx6: Swap Ethernet interrupt defines

[Guenter Roeck <linux@roeck-us.net>]

On Fri, Mar 09, 2018 at 05:47:16PM +0000, Peter Maydell wrote:
> On 8 March 2018 at 18:28, Bill Paul <wpaul@windriver.com> wrote:
> > Anyway, this means that the only reason older Linux kernels worked in QEMU
> > with the broken interrupt configuration is that they also registered a handler
> > on vector 151 (119). Even though QEMU could not send events via GPIO6, it was
> > mistakenly sending them via vector 151, so it looked like things worked. On
> > real hardware, the older kernels would have gotten their interrupts via GPIO6
> > and also worked. The older kernels would _not_ work if you fix QEMU because
> > now they would never get interrupts on either vector, unless you fudge things
> > so that the ENET module triggers both vector 150 and the vector for GPIO6 in
> > the GIC or patch them to back out the erratum 6678 workaround as later kernels
> > do.
> 
> Thanks for that really useful writeup. So if I understand correctly
> we have several choices here:
> 
>  (1) we could implement a model of the IOMUX block that is at least
>  sufficient to support guests that configure it to route the ENET interrupt
>  line to a GPIO pin. Then we could apply this patch that fixes the ENET
>  line definitions. Old kernels would continue to work (for the same
>  reason they worked on hardware), and new ones would work now too.
>  This is in some ways the preferred option, but it's possibly a lot
>  of code and we're nearly in freeze for 2.12.
> 
>  (2) we could leave everything as it is for 2.12. This would mean that
>  at least we don't regress setups that used to work on older QEMU versions.
>  Downside is that we wouldn't be able to run Linux v4.15+, or other
>  guest OSes that don't have the bug that older Linux kernels do.
>  (Presumably we'd only do this on the understanding that we were going
>  to go down route (1) for 2.13.)
> 
>  (3) we could apply this patch for 2.12. Linux v4.15+ now works, as
>  do other guest OSes that use the ENET interrupt. v4.1 and older Linux
>  guests that used to boot in QEMU stop doing so, and 4.2-4.9 boot but
>  lose the ethernet device support. Perhaps for 2.13 we might
>  take route (1) to make those older guests start working again.
> 
> Do I have that right?
> 
Pretty much.

> None of these options seems especially palatable to me, so we're
> choosing the lesser evil, I think... (unless somebody wants to say
> that option (1) would be 20 lines of code and here's the patch :-))
> I guess in the absence of (1) that (3) is better than (2) ?
> 

I would prefer (2). This is what I decided to use in my "local"
version of qemu. Older versions of Linux can be fixed by applying one
(4.2..4.9) or two (4.1 and older) upstream patches; anyone interested
running those kernels in qemu with Ethernet working should apply those
patches (or, alternatively, provide a patch adding IOMUX support to
qemu).

Thanks,
Guenter