On Tue, Oct 14, 2014 at 03:20:46PM +0200, Arnd Bergmann wrote:

On Tuesday 14 October 2014 16:07:38 Laurent Pinchart wrote:

quoted

On Tuesday 23 September 2014 09:44:25 Arnd Bergmann wrote:

quoted

On Tuesday 23 September 2014 09:02:39 Thierry Reding wrote:

quoted

quoted

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. And while it's true that earlier attempts
to put this into the core were rejected, I think there's still value in
proposing it again. The alternative proposed here is similarly close to
the core and needs to duplicated for every architecture. That itself is
to me a strong indication that this really does belong in the core.

I think initially this was proposed to become part of really_probe() and
I still think that's where it belongs. There's precedent for it with the
pinctrl_bind_pins() call, though it seems like Greg regrets allowing
that into the core. Perhaps if really_probe() is "too core", then
platform_drv_probe() would be a better candidate?

Thierry
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