linux-kernel - Re: [PATCH v3 12/29] arm_mpam: Add helpers for managing the locking around the mon

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Message-ID: <20251024184344.000036f6@huawei.com>
Date: Fri, 24 Oct 2025 18:43:44 +0100
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: James Morse <james.morse@....com>
CC: <linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
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Subject: Re: [PATCH v3 12/29] arm_mpam: Add helpers for managing the locking
 around the mon_sel registers

On Fri, 17 Oct 2025 18:56:28 +0000
James Morse <james.morse@....com> wrote:

> The MSC MON_SEL register needs to be accessed from hardirq for the overflow
> interrupt, and when taking an IPI to access these registers on platforms
> where MSC are not accessible from every CPU. This makes an irqsave
> spinlock the obvious lock to protect these registers. On systems with SCMI
> or PCC mailboxes it must be able to sleep, meaning a mutex must be used.
> The SCMI or PCC platforms can't support an overflow interrupt, and
> can't access the registers from hardirq context.
> 
> Clearly these two can't exist for one MSC at the same time.
> 
> Add helpers for the MON_SEL locking. For now, use a irqsave spinlock and
> only support 'real' MMIO platforms.
> 
> In the future this lock will be split in two allowing SCMI/PCC platforms
> to take a mutex. Because there are contexts where the SCMI/PCC platforms
> can't make an access, mpam_mon_sel_lock() needs to be able to fail. Do
> this now, so that all the error handling on these paths is present. This
> allows the relevant paths to fail if they are needed on a platform where
> this isn't possible, instead of having to make explicit checks of the
> interface type.
> 
> Tested-by: Fenghua Yu <fenghuay@...dia.com>
> Signed-off-by: James Morse <james.morse@....com>
> ---
> Change since v1:
>  * Made accesses to outer_lock_held READ_ONCE() for torn values in the failure
>    case.
Guess that went away. I'd prune the old version log or add something to indicate
it did in a later version log.

One stray change inline otherwise seems fine
Reviewed-by: Jonathan Cameron <jonathan.cameron@...wei.com>


> ---
>  drivers/resctrl/mpam_devices.c  |  3 ++-
>  drivers/resctrl/mpam_internal.h | 38 +++++++++++++++++++++++++++++++++
>  2 files changed, 40 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 910bb6cd5e4f..35011d3e8f1e 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -738,6 +738,7 @@ static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev)
>  
>  	mutex_init(&msc->probe_lock);
>  	mutex_init(&msc->part_sel_lock);
> +	mpam_mon_sel_lock_init(msc);
>  	msc->id = pdev->id;
>  	msc->pdev = pdev;
>  	INIT_LIST_HEAD_RCU(&msc->all_msc_list);
> @@ -822,7 +823,7 @@ static void mpam_enable_once(void)
>  				      "mpam:online");
>  
>  	/* Use printk() to avoid the pr_fmt adding the function name. */
> -	printk(KERN_INFO, "MPAM enabled with %u PARTIDs and %u PMGs\n",
> +	printk(KERN_INFO "MPAM enabled with %u PARTIDs and %u PMGs\n",
Move this to original patch.

>  	       mpam_partid_max + 1, mpam_pmg_max + 1);
>  }