Re: [PATCH 5/7 v6] ARM: l2c: parse 'cache-size' and 'cache-sets' properties

Arnd Bergmann
2014-09-08T13:16:59Z

On Monday 08 September 2014 14:36:48 Linus Walleij wrote:
> On Mon, Sep 8, 2014 at 2:20 PM, Arnd Bergmann <arnd@arndb.de> 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.

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

Arnd