Re: [PATCH 6/7 v6] ARM: l2x0: calculate associativity from ePAPR cache props

Linus Walleij
2014-09-08T12:43:55Z

On Mon, Sep 8, 2014 at 2:15 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Monday 08 September 2014 13:38:05 Linus Walleij wrote:
>> static void __init l2x0_cache_size_of_parse(const struct device_node *np,
>> u32 *aux_val, u32 *aux_mask,
>> - u32 max_way_size)
>> + u32 max_way_size,
>> + u32 max_associativity)
>> {
>> u32 mask = 0, val = 0;
>> u32 size = 0, sets = 0;
>> u32 way_size = 0, way_size_bits = 1;
>> + u32 linesize = 0;
>> + u32 assoc = 0;
>>
>> of_property_read_u32(np, "cache-size", &size);
>> of_property_read_u32(np, "cache-sets", &sets);
>> + of_property_read_u32(np, "cache-line-size", &linesize);
>>
>
> If I read ePAPR correctly, you have to read "cache-block-size" as well, and
> use that as a fallback if "cache-line-size" is not provided. The line size
> is only required to be in the DT if it's different from the block size.

OK fixed this...

>> if (way_size > max_way_size) {
>> - pr_warn("L2C: way size %dKB is too large\n", way_size >> 10);
>> + pr_warn("L2C OF: way size %dKB is too large\n", way_size >> 10);
>> return;
>> }
>>
>> + /* All these l2 caches have the same line size actually */
>> + if (!linesize)
>> + linesize = CACHE_LINE_SIZE;
>> + if (linesize != CACHE_LINE_SIZE)
>> + pr_warn("L2C OF: DT supplied line size %d bytes does "
>> + "not match hardware line size of %d bytes\n",
>> + linesize,
>> + CACHE_LINE_SIZE);
>
> and CACHE_LINE_SIZE seems to be what ePAPR calls cache-block-size, i.e.
> the fundamental unit of cache management as seen from the CPU, independent
> of how it is physically organized.

Yeah. Well, actually that means the variable in cache-l2x0.c should
be renamed from CACHE_LINE_SIZE to CACHE_BLOCK_SIZE
if we go with that terminology, right? That is the real change that
is needed to make terminology consistent then.

>> + /*
>> + * This cache is set associative. By increasing associativity
>> + * we increase the number of blocks per set. Usually this
>> + * quickly hits the roof at 8 or 16 ways of associativity.
>> + */
>> + assoc = way_size / linesize;
>> + if (assoc > max_associativity)
>> + assoc = max_associativity;
>> +
>
> I don't see what this helps us. Doesn't this silently try to fix up
> incorrect device trees?

I will look closer... But whatever comes out of the calculation
is always way about 8 or even 16. Probably my arithmetics
get it wrong because of terminology confusion. With the
definitions from your previous mail, maybe it all ends up
correctly. I'll hack around a bit and see.

>> + } else {
>> + if (sets == 1)
>> + /* Direct-mapped cache */
>> + assoc = 1;
>> + val |= (assoc << L2X0_AUX_CTRL_ASSOC_SHIFT);
>> + }
>
> This looks wrong: isn't "sets==1" fully associative rather than
> direct-mapped?

Yep. I'll fix.

Yours,
Linus Walleij