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

On Mon, Sep 8, 2014 at 2:20 PM, Arnd Bergmann [off-list ref] wrote:

On Monday 08 September 2014 13:38:04 Linus Walleij wrote:

quoted

+       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

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

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.

Florian: what was your interpretation?

Yours,
Linus Walleij