On Thu, Jul 10, 2014 at 11:32:16PM +0100, Olav Haugan wrote: > On 7/9/2014 3:54 AM, Will Deacon wrote: > > On Wed, Jul 09, 2014 at 02:07:38AM +0100, Olav Haugan wrote: > >> So how does an algorithm figure this out in both my examples? The > >> algorithm would have to know about both (all) bus masters and their > >> stream IDs for a specific SMMU. If the algorithm operates on the set of > >> stream IDs for one bus master at a time the algorithm has no way of > >> knowing which bits can be ignored since it doesn't know the value of the > >> other stream IDs for the other bus masters and thus could potentially > >> create a mask that could cause a stream ID to match in two different > >> entries. > > > > Complete knowledge of the system topology (i.e. all bus masters) is a > > requirement for being able to configure the SMMU correctly if you want to > > guarantee that you don't have SMR aliasing issues. > > So you agree that an algorithm needs to know about all the bus > masters/stream IDs for a specific IOMMU before it can figure out the > StreamID masks and how many SMRs can be allocated to a specific bus > master? Andreas's algorithm does not know about the other bus > masters/stream IDs. It operates on one bus master at a time. Right, but it can certainly be improved. There are certain things you can do without complete knowledge, as I mentioned previously (if you can have densely packed power-of-2 aligned/sized regions). > >>>> I am not familiar with Andreas's proposal. Do you have a link? > >>> > >>> http://marc.info/?l=linux-arm-kernel&m=139110598005846&w=2 > >> > >> Unless I am mistaken the algorithm works on one bus master at a time. I > >> don't think that will work. > > > > IIRC, it works for densely packed SIDs on the master, so it tries to build > > up power-of-2 sized groups for that master then mops up the rest with > > individual entries. > > I ran the algorithm through a few trivial cases: > > 1) > Stream IDs: 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28 > Number of SMRs: 9 > > In this case the algorithm decided to set mask to 0 for all entries > using up 8 of the SMRs. Well think about what it's doing... we don't know about SID 0x20, for example so there's not much it can do. > 2) Same Stream IDs but only 2 SMRs. > The algorithm gave an error saying I did not have enough SMRs. There's a reason this didn't get merged, and it would be great if you could try to improve the situation ;). Will -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
