linux-kernel - Re: [PATCH 15/33] arm_mpam: Add helpers for managing the locking around the mon

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Message-ID: <5e864c02-381f-4128-b539-2348c149a459@nvidia.com>
Date: Wed, 12 Nov 2025 19:52:33 -0800
From: Fenghua Yu <fenghuay@...dia.com>
To: Ben Horgan <ben.horgan@....com>, james.morse@....com
Cc: amitsinght@...vell.com, baisheng.gao@...soc.com,
 baolin.wang@...ux.alibaba.com, bobo.shaobowang@...wei.com,
 carl@...amperecomputing.com, catalin.marinas@....com, dakr@...nel.org,
 dave.martin@....com, david@...hat.com, dfustini@...libre.com,
 gregkh@...uxfoundation.org, gshan@...hat.com, guohanjun@...wei.com,
 jeremy.linton@....com, jonathan.cameron@...wei.com, kobak@...dia.com,
 lcherian@...vell.com, lenb@...nel.org, linux-acpi@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 lpieralisi@...nel.org, peternewman@...gle.com, quic_jiles@...cinc.com,
 rafael@...nel.org, robh@...nel.org, rohit.mathew@....com,
 scott@...amperecomputing.com, sdonthineni@...dia.com, sudeep.holla@....com,
 tan.shaopeng@...itsu.com, will@...nel.org, xhao@...ux.alibaba.com,
 Shaopeng Tan <tan.shaopeng@...fujitsu.com>
Subject: Re: [PATCH 15/33] arm_mpam: Add helpers for managing the locking
 around the mon_sel registers



On 11/7/25 04:34, Ben Horgan wrote:
> From: James Morse <james.morse@....com>
> 
> 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>
> Reviewed-by: Jonathan Cameron <jonathan.cameron@...wei.com>
> Tested-by: Shaopeng Tan <tan.shaopeng@...fujitsu.com>
> Tested-by: Peter Newman <peternewman@...gle.com>
> Signed-off-by: James Morse <james.morse@....com>
> Signed-off-by: Ben Horgan <ben.horgan@....com>

Reviewed-by: Fenghua Yu <fenghuay@...dia.com>

Thanks.

-Fenghua