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 -- To unsubscribe from this list: send the line "unsubscribe linux-samsung-soc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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