On Wed, May 18, 2016 at 04:55:45PM +0100, Andre Przywara wrote: > Hi, > > .... > > >> + > >> +/* generate a mask that covers 1024 interrupts with <b> bits per IRQ */ > >> +#define VGIC_ADDR_IRQ_MASK(b) GENMASK_ULL(ilog2(b) + ilog2(1024) - \ > >> + ilog2(BITS_PER_BYTE) - 1, 0) > >> +#define VGIC_ADDR_TO_INTID(addr, bits) (((addr) & VGIC_ADDR_IRQ_MASK(bits)) * \ > >> + 64 / (bits) / 8) > > > > In the comment we end up adding here, can we also describe why > > (addr & <magic mask>) * <magic 64> / (bits) / <BITS_PER_BYTE OR BYTES_PER_ULL> > > gives us what we need, because I don't get it. > > The reason is: we deal with 8 bits per byte, but have > bits-per-interrupts values bigger than 8. Doing the maths in floating > point arithmetic would work fine: > > (float)(addr & mask) * (8.0 / bits_per_IRQ) > > So would this comment make sense? > > /* > * Since we can have more than 8 bits per interrupt, we can't use > * "8 / bpi" as a multiplicand directly, so we use a > * fixed-point-arithmetic version of it tailored to cover at most 64 > * bits per IRQ. > */ > Something like this certainly helps, here's another version which is easier for me to understand, but you can take your pick: /* * (addr & mask) gives us the byte offset for the INT ID, so we want to * divide this with 'bytes per irq' to get the INT ID, which is given * by '(bits) / 8'. But we do this with fixed-point-arithmetic and * take advantage of the fact that division by a fraction equals * multiplication with the inverted fraction, and scale up both the * numerator and denominator with 8 to support at most 64 bits per IRQ: */ At least I think we all agree that the approach works by now. -Christoffer -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
