[Nouveau] [PATCH 1/6] platform: specify the IOMMU physical translation bit

[Terje Bergstrom]

On 04/16/2015 11:26 PM, Alexandre Courbot wrote:
> Instead of this function, you should probably use the data field of
> struct of_device_id. Define a local struct called, say,
> nouveau_platform_params containing (for now) a single iommu_addr_bit
> field and instanciate one for each entry of nouveau_platform_match.
> Then you can cast match->data and retrieve the field directly instead
> of using strcmp.
>
> I'd say this is then simple enough to do directly in
> nouveau_platform_probe_iommu() instead of having a function dedicated
> to it. It is also safer because when we add a new chip, we have to
> update nouveau_platform_match but might very well forget about your
> function, and will end up with bit 0 being set on all our GPU
> addresses and endless hours of fun figuring out how this happened. :)
>
> While I am at it: how about defining this as a u64 mask to set/unset
> on GPU addresses instead of just a bit? This is more flexible and
> again safer in case someone "omits" to specify the correct bit for a
> chip.
Wouldn't u64 be as dangerous? We'd be just messing up with address in 
another way.

Another way of expressing this information would be defining number of 
physical(*) address bits. IOMMU bit is the bit after physical address bits.

Terje

* Physical here means after GPU MMU translation, i.e. either physical or 
IOMMU address. Somebody needs to come up with a term to cover that.