linux-kernel - Re: [PATCH v2 37/45] arm_mpam: resctrl: Add empty definitions for assorted resctrl functions

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Message-ID: <20260106144835.00007dca@huawei.com>
Date: Tue, 6 Jan 2026 14:48:35 +0000
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: Ben Horgan <ben.horgan@....com>
CC: <amitsinght@...vell.com>, <baisheng.gao@...soc.com>,
	<baolin.wang@...ux.alibaba.com>, <carl@...amperecomputing.com>,
	<dave.martin@....com>, <david@...nel.org>, <dfustini@...libre.com>,
	<fenghuay@...dia.com>, <gshan@...hat.com>, <james.morse@....com>,
	<kobak@...dia.com>, <lcherian@...vell.com>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
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	<joey.gouly@....com>, <suzuki.poulose@....com>, <kvmarm@...ts.linux.dev>
Subject: Re: [PATCH v2 37/45] arm_mpam: resctrl: Add empty definitions for
 assorted resctrl functions

On Fri, 19 Dec 2025 18:11:39 +0000
Ben Horgan <ben.horgan@....com> wrote:

> From: James Morse <james.morse@....com>
> 
> A few resctrl features and hooks need to be provided, but aren't needed or
> supported on MPAM platforms.
> 
> resctrl has individual hooks to separately enable and disable the
> closid/partid and rmid/pmg context switching code. For MPAM this is all the
> same thing, as the value in struct task_struct is used to cache the value
> that should be written to hardware. arm64's context switching code is
> enabled once MPAM is usable, but doesn't touch the hardware unless the
> value has changed.
> 
> For now event configuration is not supported, and can be turned off by
> returning 'false' from resctrl_arch_is_evt_configurable().
> 
> The new io_alloc feature is not supported either, always return false from
> the enable helper to indicate and fail the enable.
> 
> Add this, and empty definitions for the other hooks.
> 
> Signed-off-by: James Morse <james.morse@....com>
> Signed-off-by: Ben Horgan <ben.horgan@....com>

FWIW given I didn't check what these all are, just that they are indeed
stubs that do the obvious things.

Reviewed-by: Jonathan Cameron <jonathan.cameron@...wei.com>
> ---
>  drivers/resctrl/mpam_resctrl.c | 27 +++++++++++++++++++++++++++
>  include/linux/arm_mpam.h       |  9 +++++++++
>  2 files changed, 36 insertions(+)
> 
> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
> index 84f517cb047a..059148c38a38 100644
> --- a/drivers/resctrl/mpam_resctrl.c
> +++ b/drivers/resctrl/mpam_resctrl.c
> @@ -93,6 +93,23 @@ bool resctrl_arch_mon_capable(void)
>  	return exposed_mon_capable;
>  }
>  
> +bool resctrl_arch_is_evt_configurable(enum resctrl_event_id evt)
> +{
> +	return false;
> +}
> +
> +void resctrl_arch_mon_event_config_read(void *info)
> +{
> +}
> +
> +void resctrl_arch_mon_event_config_write(void *info)
> +{
> +}
> +
> +void resctrl_arch_reset_rmid_all(struct rdt_resource *r, struct rdt_mon_domain *d)
> +{
> +}
> +
>  bool resctrl_arch_get_cdp_enabled(enum resctrl_res_level rid)
>  {
>  	switch (rid) {
> @@ -1131,6 +1148,16 @@ int resctrl_arch_mbm_cntr_assign_set(struct rdt_resource *r, bool enable)
>  	return 0;
>  }
>  
> +int resctrl_arch_io_alloc_enable(struct rdt_resource *r, bool enable)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
> +bool resctrl_arch_get_io_alloc_enabled(struct rdt_resource *r)
> +{
> +	return false;
> +}
> +
>  static int mpam_resctrl_control_init(struct mpam_resctrl_res *res,
>  				     enum resctrl_res_level type)
>  {
> diff --git a/include/linux/arm_mpam.h b/include/linux/arm_mpam.h
> index 86d5e326d2bd..f92a36187a52 100644
> --- a/include/linux/arm_mpam.h
> +++ b/include/linux/arm_mpam.h
> @@ -67,6 +67,15 @@ struct rdt_resource;
>  void *resctrl_arch_mon_ctx_alloc(struct rdt_resource *r, enum resctrl_event_id evtid);
>  void resctrl_arch_mon_ctx_free(struct rdt_resource *r, enum resctrl_event_id evtid, void *ctx);
>  
> +/*
> + * The CPU configuration for MPAM is cheap to write, and is only written if it
> + * has changed. No need for fine grained enables.
> + */
> +static inline void resctrl_arch_enable_mon(void) { }
> +static inline void resctrl_arch_disable_mon(void) { }
> +static inline void resctrl_arch_enable_alloc(void) { }
> +static inline void resctrl_arch_disable_alloc(void) { }
> +
>  static inline unsigned int resctrl_arch_round_mon_val(unsigned int val)
>  {
>  	return val;