On 09/08/2014 06:16 AM, Arnd Bergmann wrote: > On Monday 08 September 2014 14:36:48 Linus Walleij wrote: >> On Mon, Sep 8, 2014 at 2:20 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote: >>> On Monday 08 September 2014 13:38:04 Linus Walleij wrote: >>>> + of_property_read_u32(np, "cache-size", &size); >>>> + of_property_read_u32(np, "cache-sets", &sets); >>>> + >>>> + if (!size || !sets) >>>> + return; >>>> + >>>> + way_size = size / sets; >>> >>> Going back to this one: Isn't (size / sets) the set-size rather >>> than the way-size? >>> >>> After we discussed this on IRC, I had expected a calculation like >>> >>> set_size = size / sets; >>> ways = set_size / line_size; >>> way_size = size / ways; >> >> First: in this PB1176 case: >> >> set_size = 128K/8 = 16K >> ways = 16K/32 = 512 bytes >> way_size = 128K/512 = 128 bytes > > I guess we should first try to find the right units so we know what > we are talking about. I was under the impression that the set size > is the number of bytes in a set, while 'ways' is counting the > associativity and has no unit. I agree with that. > > The last line also seems to incorrectly computed. Dividing 128KB > by 512 bytes should be 256 (no unit). > >> Well maybe it's the ARM reference manual internal lingo that >> is actually causing the confusion here. It will say something >> like: >> >> [19:17] Way-size 3’b000 = Reserved, internally mapped to 16KB >> 3’b001 = 16KB, this is the default value >> 3’b010 = 32KB >> 3’b011 = 64KB >> 3’b100 = 128KB >> 3’b101 = 256KB >> 3’b110 to 3’b111 = Reserved, internally mapped to 256 KB >> >> OK way-size ... is in the 16 thru 256KB range, which fits nicely >> with set size incidentally. And also corresponds to current >> comments in the code such as this from >> arch/arm/mach-realview/realview_pb1176.c: >> >> #ifdef CONFIG_CACHE_L2X0 >> /* >> * The PL220 needs to be manually configured as the hardware >> * doesn't report the correct sizes. >> * 128kB (16kB/way), 8-way associativity, event monitor and >> * parity enabled, ignore share bit, no force write allocate >> * Bits: .... ...0 0111 0011 0000 .... .... .... >> */ >> l2x0_init(__io_address(REALVIEW_PB1176_L220_BASE), 0x00730000, >> 0xfe000fff); >> #endif > > 16KB way-size seems realistic, yes. > >> I can add a comment explaining that ARMs terminology does >> not match the academic terminology or something, and say that >> the thing we poke into "way-size" is actually "set size", if we agree >> that is what we're seeing here. > > Let's see again: 16KB way-size * 8 ways = 128KB, that seems correct. > 16KB way-size / 32B line-size = 512 sets, that also seems realistic. > 128KB cache-size / 512 sets = 256B set-size. 256B / 32B = 8 ways, > so everything fits together. > > Now in the code: > > + of_property_read_u32(np, "cache-size", &size); > > 131072 > > + of_property_read_u32(np, "cache-sets", &sets); > > 512 > > + > + if (!size || !sets) > + return; > + > + way_size = size / sets; > > 256 -> impossible. > > > set_size = size / sets; > > 256 > > ways = set_size / line_size; > > 8 > > way_size = size / ways; > > 16KB -> ok So what we are missing right now is just a 'cache-line-size' property to get the maths right. -- Florian -- 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
