Re: [PATCH v3 27/29] arm_mpam: Add helper to reset saved mbwu state

[Jonathan Cameron <jonathan.cameron@huawei.com>]

On Fri, 17 Oct 2025 18:56:43 +0000
James Morse <james.morse@arm.com> wrote:

> resctrl expects to reset the bandwidth counters when the filesystem
> is mounted.
> 
> To allow this, add a helper that clears the saved mbwu state. Instead
> of cross calling to each CPU that can access the component MSC to
> write to the counter, set a flag that causes it to be zero'd on the
> the next read. This is easily done by forcing a configuration update.
> 
> Signed-off-by: James Morse <james.morse@arm.com>
> Reviewed-by: Fenghua Yu <fenghuay@nvdia.com>
> Tested-by: Fenghua Yu <fenghuay@nvidia.com>
A couple of little places in here where I think moving things back to earlier
patches would reduce churn a little which is always nice for reviewers.

Either way
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>

> ---
> Changes since v2:
>  * Switched to guard and fixed non _srcu list walker.
>  * Made a comment about what is proteted by which lock a list.
> ---
>  drivers/resctrl/mpam_devices.c  | 46 ++++++++++++++++++++++++++++++++-
>  drivers/resctrl/mpam_internal.h |  7 ++++-
>  2 files changed, 51 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index c207a6d2832c..89d4f42168ed 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c

>  	/*
>  	 * Read the existing configuration to avoid re-writing the same values.
>  	 * This saves waiting for 'nrdy' on subsequent reads.
> @@ -1090,7 +1100,10 @@ static void __ris_msmon_read(void *arg)
>  	read_msmon_ctl_flt_vals(m, &cur_ctl, &cur_flt);
>  	clean_msmon_ctl_val(&cur_ctl);
>  	gen_msmon_ctl_flt_vals(m, &ctl_val, &flt_val);
> -	if (cur_flt != flt_val || cur_ctl != (ctl_val | MSMON_CFG_x_CTL_EN))
> +	config_mismatch = cur_flt != flt_val ||
> +			  cur_ctl != (ctl_val | MSMON_CFG_x_CTL_EN);

Push back to earlier patch perhaps?  I guess someone might ask why you don't do it
inline, but to me it seems complex enough that I doubt they will.
Nice to reduce the churn where it is easy to do.

> +
> +	if (config_mismatch || reset_on_next_read)
>  		write_msmon_ctl_flt_vals(m, ctl_val, flt_val);
>  
>  	switch (m->type) {
> @@ -1242,6 +1255,37 @@ int mpam_msmon_read(struct mpam_component *comp, struct mon_cfg *ctx,
>  	return err;
>  }


> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
> index ff38b4bbfc2b..6632699ae814 100644
> --- a/drivers/resctrl/mpam_internal.h
> +++ b/drivers/resctrl/mpam_internal.h
> @@ -204,10 +204,14 @@ struct mon_cfg {
>  
>  /*
>   * Changes to enabled and cfg are protected by the msc->lock.
> - * Changes to prev_val and correction are protected by the msc's mon_sel_lock.
> + * The msc's mon_sel_lock protects:
Nice to push this formatting change back to earlier patch so this
becomes a one line change.

> + * - reset_on_next_read
> + * - prev_val
> + * - correction
>   */
>  struct msmon_mbwu_state {
>  	bool		enabled;
> +	bool		reset_on_next_read;
>  	struct mon_cfg	cfg;