Re: [PATCH RFC 02/15] cacheinfo: Expose the code to generate a cache-id from a device_node

[Yin Li]

On 8/26/2026 3:11 AM, Drew Fustini wrote:
> On Tue, Aug 11, 2026 at 09:30:31PM +0800, Yin Li wrote:
> > From: James Morse <james.morse@xxxxxxx>
> >
> > The MPAM driver identifies caches by id for use with resctrl. It
> > needs to know the cache-id when probe-ing, but the value isn't set
> > in cacheinfo until device_initcall(). Even after device_initcall(),
> > the cache-id is only available if at least one CPU associated with
> > the cache is online.
> >
> > Instead of making the driver wait, expose the code that generates the
> > cache-id. The parts of the MPAM driver that run early can use this to
> > set up the resctrl structures before cacheinfo is ready in
> > device_initcall().
>
> This patch would also be helpful for resctrl support on RISC-V, too. The
> probe function in the CBQRI capacity allocation platform driver [1] has
> the same problem. The current solution is to scan the online CPUs for a
> cacheinfo leaf whose fw_token is the cache node from the phandle.
> However, that does not work if no cpu sharing that cache is online yet.

Hi Drew,

Thanks for the review.

Good to know that cache_of_calculate_id() could help with the RISC-V
CBQRI driver as well. Since the function is guarded by CONFIG_OF in
cacheinfo.h, it should be available on DT-based RISC-V platforms
without any issue. You're welcome to use it.

> [..]
> > +u32 cache_of_calculate_id(struct device_node *cache_node)
> >   {
> >   	struct device_node *cpu;
> >   	u32 min_id = ~0;
> > @@ -238,15 +237,23 @@ static void cache_of_set_id(struct cacheinfo *this_leaf,
> >   		id = arch_compact_of_hwid(id);
> >   		if (FIELD_GET(GENMASK_ULL(63, 32), id)) {
> >   			of_node_put(cpu);
> > -			return;
> > +			return ~0;
>
> Maybe this should be U32_MAX instead?

Regarding the ~0 vs U32_MAX point:
You're right that U32_MAX is more appropriate than ~0 here. This is
addressed in patch 06/15 ("arm_mpam: Fix cache ID sentinel from ~0UL
to U32_MAX to match u32 return type").
https://lore.kernel.org/all/20260811-mpam-resctrl-dt-knp-support-v1-6-ea6397bead59@xxxxxxxxxxxxxxxx/

The fix is in a separate patch because this patch is based on James
Morse's original unmerged work — I kept my fixes separate for easier
review. The plan is to fold them back in once James has responded.

> Thanks,
> Drew
>
> [1] https://lore.kernel.org/all/20260729-dfustini-atl-sc-cbqri-dt-v6-8-7c22b05d461b@xxxxxxxxxx/

--
Thx and BRs,
Yin