Re: [PATCH] iommu/dma: Resurrect the "forcedac" option

[John Garry]

On 08/03/2021 13:08, Robin Murphy wrote:
> On 2021-03-05 17:41, John Garry wrote:
> > On 05/03/2021 16:32, Robin Murphy wrote:
> > > In converting intel-iommu over to the common IOMMU DMA ops, it quietly
> > > lost the functionality of its "forcedac" option. Since this is a handy
> > > thing both for testing and for performance optimisation on certain
> > > platforms, reimplement it under the common IOMMU parameter namespace.
> > >
> > > For the sake of fixing the inadvertent breakage of the Intel-specific
> > > parameter, remove the dmar_forcedac remnants and hook it up as an alias
> > > while documenting the transition to the new common parameter.
> >
> > Do you think that having a kconfig option to control the default for 
> > this can help identify the broken platforms which rely on forcedac=0? 
> > But seems a bit trivial for that, though.
>
> I think it's still a sizeable can of worms - unlike, say, 
> ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT, we can't actually tell when things 
> have gone awry and explicitly call it out. While I was getting the 
> dma-ranges right on my Juno, everything broke differently - the SATA 
> controller fails gracefully; the ethernet controller got the kernel tied 
> up somewhere (to the point that the USB keyboard died) once it tried to 
> brink up the link, but was at least spewing regular timeout backtraces 
> that implicated the networking layer; having an (unused) NVMe plugged in 
> simply wedged the boot process early on with no hint whatsoever of why.
>
> TBH I'm not really sure what the best way forward is in terms of trying 
> to weed out platforms that would need quirking.

I was more thinking of an unstable TEST config, like 
DEBUG_TEST_DRIVER_REMOVE. So we know that this particular config breaks 
many platforms. But at least those in the know can turn it on locally 
and detect and fix issues, and strive towards having a platform for 
which it works.

But then it does become a little harder to justify such a config when we 
can enable via commadline.

> Our discussion just 
> reminded me of this option and that it had gone AWOL, so bringing it 
> back to be potentially *some* use to everyone seems justifiable on its own.

Of course.

Cheers,
John