linux-kernel - Re: [PATCH 10/33] arm_mpam: Add probe/remove for mpam msc driver and kbuild boiler plate

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Message-ID: <6a050b35-1b4d-4ea1-aa40-974aafa89b52@arm.com>
Date: Fri, 5 Sep 2025 19:49:08 +0100
From: James Morse <james.morse@....com>
To: Ben Horgan <ben.horgan@....com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-acpi@...r.kernel.org,
 devicetree@...r.kernel.org
Cc: shameerali.kolothum.thodi@...wei.com,
 D Scott Phillips OS <scott@...amperecomputing.com>,
 carl@...amperecomputing.com, lcherian@...vell.com,
 bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
 baolin.wang@...ux.alibaba.com, Jamie Iles <quic_jiles@...cinc.com>,
 Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com,
 dfustini@...libre.com, amitsinght@...vell.com,
 David Hildenbrand <david@...hat.com>, Rex Nie <rex.nie@...uarmicro.com>,
 Dave Martin <dave.martin@....com>, Koba Ko <kobak@...dia.com>,
 Shanker Donthineni <sdonthineni@...dia.com>, fenghuay@...dia.com,
 baisheng.gao@...soc.com, Jonathan Cameron <jonathan.cameron@...wei.com>,
 Rob Herring <robh@...nel.org>, Rohit Mathew <rohit.mathew@....com>,
 Rafael Wysocki <rafael@...nel.org>, Len Brown <lenb@...nel.org>,
 Lorenzo Pieralisi <lpieralisi@...nel.org>, Hanjun Guo
 <guohanjun@...wei.com>, Sudeep Holla <sudeep.holla@....com>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Catalin Marinas <catalin.marinas@....com>,
 Will Deacon <will@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Danilo Krummrich <dakr@...nel.org>
Subject: Re: [PATCH 10/33] arm_mpam: Add probe/remove for mpam msc driver and
 kbuild boiler plate

Hi Ben,

On 01/09/2025 10:11, Ben Horgan wrote:
> On 8/22/25 16:29, James Morse wrote:
>> Probing MPAM is convoluted. MSCs that are integrated with a CPU may
>> only be accessible from those CPUs, and they may not be online.
>> Touching the hardware early is pointless as MPAM can't be used until
>> the system-wide common values for num_partid and num_pmg have been
>> discovered.
>>
>> Start with driver probe/remove and mapping the MSC.

>> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
>> new file mode 100644
>> index 000000000000..07e0f240eaca
>> --- /dev/null
>> +++ b/drivers/resctrl/mpam_internal.h
>> @@ -0,0 +1,62 @@

>> +struct mpam_msc {
>> +	/* member of mpam_all_msc */
>> +	struct list_head        glbl_list;
>> +
>> +	int			id;
>> +	struct platform_device *pdev;
>> +
>> +	/* Not modified after mpam_is_enabled() becomes true */
>> +	enum mpam_msc_iface	iface;
>> +	u32			pcc_subspace_id;
>> +	struct mbox_client	pcc_cl;
>> +	struct pcc_mbox_chan	*pcc_chan;
>> +	u32			nrdy_usec;
>> +	cpumask_t		accessibility;
>> +
>> +	/*
>> +	 * probe_lock is only take during discovery. After discovery these
>> +	 * properties become read-only and the lists are protected by SRCU.
>> +	 */
>> +	struct mutex		probe_lock;
>> +	unsigned long		ris_idxs[128 / BITS_PER_LONG];

> Why is this sized this way? RIS_MAX is 4 bits and so there are at most
> 16 RIS per msc.

Hmmm, lost in time - I agree with the 16 reasoning. Fixed.
(It's likely due to RES0 space above the field - but that has been filled in with other
 stuff since then. RIS was added as a 'backward compatible feature' - I was wary of them
 extending it)


>> +	u32			ris_max;
>> +
>> +	/* mpam_msc_ris of this component */
>> +	struct list_head	ris;
>> +
>> +	/*
>> +	 * part_sel_lock protects access to the MSC hardware registers that are
>> +	 * affected by MPAMCFG_PART_SEL. (including the ID registers that vary
>> +	 * by RIS).
>> +	 * If needed, take msc->lock first.
>> +	 */
>> +	struct mutex		part_sel_lock;
>> +
>> +	/*
>> +	 * mon_sel_lock protects access to the MSC hardware registers that are
>> +	 * affeted by MPAMCFG_MON_SEL.
>> +	 * If needed, take msc->lock first.
>> +	 */
>> +	struct mutex		outer_mon_sel_lock;
>> +	raw_spinlock_t		inner_mon_sel_lock;
>> +	unsigned long		inner_mon_sel_flags;
>> +
>> +	void __iomem		*mapped_hwpage;
>> +	size_t			mapped_hwpage_sz;
>> +};
>> +
>> +#endif /* MPAM_INTERNAL_H */

Thanks,

James