Messages in this thread | | | Date | Tue, 31 May 2011 16:34:33 +0100 | From | Daniel Haid <> | Subject | Re: Question about iommu on x86 64 and ra deon driver. |
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> Noo.. It does, but the normal assumption of 'phys_to_virt' == > 'phys_to_bus' is > not valid anymore. Think of a buffer (swiotlb) which has a pool > of pages and when a PCI device wants a page, it hands one out. It > also has > other functionality such as 'mapping' of an already allocated page. > If the > PCI device asks the IOMMU (swiotlb) to map a page (and if you have > 'swiotlb=force' > the page provided has been allocated above 4GB and the device can > only handle > up to 32-bit),
Does the radeon driver allocate without DMA32, possibly above 4GB, ...
> then swiotlb gives out a page from its own pool. You now have > two addresses: the one from the PCI pool (swiotlb) and the one you > already > allocated.
... or does it allocate under 4GB but nevertheless get a page from the swiotlb pool?
> You are suppose to program your PCI card to read/write data to the > page provided from the IOMMU (so the swiotlb), which means that it > won't > write to the page you had allocated. Hence there are a calls, such as > 'sync_page'.. > which will copy the contents from the swiotlb page to the one you had > allocated > (or vice-versa). This is called 'bounce buffer'. > > The radeon (and nouveau) don't have the code to call the 'sync_page', > and > you wouldn't really want to do so - as it slows down the performance > of the > machine. There exists another mechanism which is to allocate the > pages > at the start, and not do mapping later one.
Why can the radeon not simply allocate addresses under 4GB and not request adresses from the iommu at all?
I assume that if you request a page from the IOMMU, you are required to do these sync_page calls (and that they get optimized away with a hardware IOMMU?).
So if the radeon uses the IOMMU but does not call sync_page even if required to the code seems to be broken. If this is indeed the case would it not be possible to simply add the sync_page calls for now (and thus fix the code), if it is not difficult, and implement the method with more performance later?
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