Re: [Linaro-mm-sig] [PATCH v3 18/19] iommu: exynos: init from dt-specific callback instead of initcall

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Arnd,

On Wednesday 17 December 2014 16:41:33 Arnd Bergmann wrote:
> On Wednesday 17 December 2014 16:39:02 Laurent Pinchart wrote:
> > On Wednesday 17 December 2014 15:27:36 Arnd Bergmann wrote:
> > > On Wednesday 17 December 2014 01:24:42 Laurent Pinchart wrote:
> > > > If we forbid the IOMMU driver from being compiled as a module can't we
> > > > just rely on deferred probing ? The bus master driver will just be
> > > > reprobed after the IOMMU gets probed, like for other devices.
> > > > 
> > > > This could fail in case the IOMMU device permanently fails to probe.
> > > > There
> > > > would be something very wrong in the system in that case, Enabling the
> > > > bus
> > > > masters totally transparently without IOMMU support could not be such
> > > > a
> > > > good idea.
> > > 
> > > I believe in the majority of cases today, the IOMMU is entirely
> > > optional.
> > > There are valid reasons for not including the IOMMU driver in the
> > > kernel,
> > > e.g. when you know that all the machines with that driver can DMA to
> > > all of their RAM and you want to avoid the overhead of IOTLB misses.
> > 
> > Should that really be controlled by compiling the IOMMU driver out,
> > wouldn't it be better to disable the IOMMU devices in DT in that case ?
> 
> It's a policy decision that should only depend on the user. Modifying the
> DT is wrong here IMHO because the device is still connected to the IOMMU
> in hardware and we should correctly represent that.

I was thinking about setting status = "disabled" on the IOMMU nodes, not 
removing the IOMMU references in the bus master nodes.

> You can normally set 'iommu=off' or 'iommu=force' on the command line
> to force it on or off rather than letting the IOMMU driver decide whether
> the device turns it on. Not enabling the IOMMU at all should really just
> behave the same way as 'iommu=off', anything else would be very confusing.

Setting iommu=off sounds fine to me. The IOMMU core would then return a "no 
IOMMU present" error instead of -EPROBE_DEFER, and probing of the bus masters 
would continue without IOMMU support.

-- 
Regards,

Laurent Pinchart