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

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

On Fri, Mar 09, 2018 at 10:53:43AM -0800, Bill Paul wrote:
> Of all the gin joints in all the towns in all the world, Guenter Roeck had to 
> walk into mine at 10:20 on Friday 09 March 2018 and say:
> 
> > 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.
> 
> There may be a 4th option.
> 
> Since older kernels work because they were looking at vector 151, you could 
> patch the imx_fec.c module so that when it signals an event for vector 150, it 
> also signals vector 151 too. This would keep newer versions of QEMU "bug-
> compatible" with older versions while also fixing support for newer Linux 
> kernels and other guests (e.g. VxWorks) that follow the hardware spec 
> correctly.
> 
> I think this would require only a small modification to this function:
> 
> static void imx_eth_update(IMXFECState *s)
> {
>     if (s->regs[ENET_EIR] & s->regs[ENET_EIMR] & ENET_INT_TS_TIMER) {
>         qemu_set_irq(s->irq[1], 1);
>     } else {
>         qemu_set_irq(s->irq[1], 0);
>     }
> 
>     if (s->regs[ENET_EIR] & s->regs[ENET_EIMR] & ENET_INT_MAC) {
>         qemu_set_irq(s->irq[0], 1);
>     } else {
>         qemu_set_irq(s->irq[0], 0);
>     }
> }
> 
> (For ENET_INT_MAC events, just signal both s->irq[0] and s->irq[1]).
> 
Now this is an excellent idea.

> This of course means signaling spurious events on vector 151, but you're doing 
> that now anyway. :)
> 
Actually, it doesn't. It looks like the first interrupt is handled, resetting
the interrupt status, and the second interrupt is never even executed. I tested
this with all kernel releases back to v3.16.

I'll send out patch v2 with this change and let Peter decide which version to
apply.

Thanks,
Guenter