Re: [RFC PATCH v3 6/7] arm: call iommu_init before of_platform_populate

[Laurent Pinchart <laurent.pinchart-ryLnwIuWjnjg/C1BVhZhaw@public.gmane.org>]

Hi Thierry,

On Tuesday 14 October 2014 17:05:29 Thierry Reding wrote:
> On Tue, Oct 14, 2014 at 06:01:58PM +0300, Laurent Pinchart wrote:
> > On Tuesday 14 October 2014 15:37:59 Thierry Reding wrote:
> >> On Tue, Oct 14, 2014 at 03:20:46PM +0200, Arnd Bergmann wrote:
> >>> On Tuesday 14 October 2014 16:07:38 Laurent Pinchart wrote:
> >>>> On Tuesday 23 September 2014 09:44:25 Arnd Bergmann wrote:
> >>>>> On Tuesday 23 September 2014 09:02:39 Thierry Reding wrote:
> >>>>>>> I see two problems with using deferred probing here:
> >>>>>>> 
> >>>>>>> - we don't actually need to defer the probing but the binding to
> >>>>>>>   the driver when no dma ops are set, but it seems silly to even
> >>>>>>>   create the device before we can find out which ops it should
> >>>>>>>   use.
> >>>>>> 
> >>>>>> What does device creation have to do with anything? Surely a device
> >>>>>> won't need IOMMU services before the device is bound to a driver.
> >>>>> 
> >>>>> The problem is that the driver will start using the IOMMU as soon
> >>>>> as it calls dma_map_*, but that happens at runtime, not necessarily
> >>>>> during the probe function.
> >>>>> 
> >>>>> So we can get into the weird situation that probe() returns success,
> >>>>> but then you can't use the device yet because you don't know whether
> >>>>> it is supposed to use an IOMMU or not.
> >>>> 
> >>>> If we want IOMMU devices to be supported by common device drivers we
> >>>> need to defer probing of the master devices, there's no doubt about
> >>>> that. Earlier approaches that hooked up into the device core code
> >>>> were rejected, but it should be possible to use bus notifiers to
> >>>> achieve the same result (with the drawback of having to register one
> >>>> notifier per bus). The BUS_NOTIFY_BIND_DRIVER notifier can then just
> >>>> return -EPROBE_DEFER when a iommus property is available and points to
> >>>> an IOMMU not registered yet. I'm not saying we have to do this, but I
> >>>> believe that at least from a technical point of view it could be done.
> >>> 
> >>> I think that fundamentally speaking, relying on notifiers for
> >>> something like this is very problematic, both in terms of
> >>> maintainability and reliability. We should really try to get the
> >>> notifiers out of the iommu handling, not put more of them in.
> >> 
> >> Agreed. Also last time I checked the driver core simply ignored the
> >> return value from notifiers, therefore this wouldn't work without
> >> changing the core either.
> >> 
> >> Still, I agree with Laurent that we really should be relying on probe
> >> deferral for probe ordering.
> > 
> > *If* we decide to support IOMMUs with real devices ;-)
> > 
> > I don't have a strong opinion on the subject. I initially thought it would
> > be a shame not to be able to use devres, until realizing that binding to
> > a DT node instead of a device meant that no unbind can ever occur.
> > Loosing dev_* support is also an annoyance though.
> 
> Binding to a DT node then also means that you can't build the driver as
> a module. And in particular for multiplatform kernels this is going to
> be a problem eventually.

It will, but other dependencies will make multiplatform kernels unpractical 
with or without IOMMUs anyway. Many platforms need clocks, timers, pin 
control, gpio, i2c and pmic support to boot to userspace. Agreed, that's not a 
valid reason to make the problem worse just for the sake of it.

-- 
Regards,

Laurent Pinchart