linux-kernel - Re: [PATCH v2 0/2] RISC-V IOMMU HPM support

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Message-ID: <D8C71AE40393A6FD+89365591-f896-4609-8b0f-8dffe85b5828@linux.spacemit.com>
Date: Mon, 9 Feb 2026 09:40:24 +0800
From: Lv Zheng <lv.zheng@...ux.spacemit.com>
To: Zong Li <zong.li@...ive.com>, tjeznach@...osinc.com, joro@...tes.org,
 will@...nel.org, robin.murphy@....com, robh@...nel.org, pjw@...nel.org,
 palmer@...belt.com, aou@...s.berkeley.edu, alex@...ti.fr,
 mark.rutland@....com, conor+dt@...nel.org, krzk@...nel.org,
 guoyaxing@...c.ac.cn, luxu.kernel@...edance.com,
 andrew.jones@....qualcomm.com, linux-kernel@...r.kernel.org,
 iommu@...ts.linux.dev, linux-riscv@...ts.infradead.org,
 linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v2 0/2] RISC-V IOMMU HPM support

On 2/8/2026 2:38 PM, Zong Li wrote:
> This series implements support for the RISC-V IOMMU hardware performance
> monitor.
> 
> The RISC-V IOMMU PMU driver is implemented as an auxiliary device driver
> created by the parent RISC-V IOMMU driver. The child driver can obtain
> resources and information from the parent device, such as the MMIO base
> address and IRQ number.
> 
> Custom event support is not included in this series and will be submitted
> as a separate series. The idea is that custom events will be defined
> through the driver data of the auxiliary driver. When the parent IOMMU
> driver creates the auxiliary device, it should pass a vendor-specific
> name as the device name in order to match the corresponding auxiliary
> device ID.
> 
> After my initial RFC and the v1 patch series were posted, similar patch
> series also appeared on the mailing list.
> 
> One was from Yaxing Guo:
> https://lore.kernel.org/all/20250915020911.1313-1-guoyaxing@bosc.ac.cn/
> 
> and another from Lv Zheng:
> https://lore.kernel.org/all/30C7AD351DB55276+a8b03dac-c5af-4034-8631-ac1c352a469f@linux.spacemit.com/

Please do not forget to Cc:
Jingyu Li <joey.li@...cemit.com>
I only provided T100 specific support in the posted series, and the
new approach of the HPM driver in the same series is composed by her.

> 
> Yaxing has agreed to pause his series, as discussed here:
> https://lore.kernel.org/linux-iommu/2ce9d8be-10b3-48dd-b99e-7358347fc171@bosc.ac.cn/
> 
> The motivation for sending this v2 version is not only to address the
> feedback and suggestions from the previous round, but also to introduce
> a major structural change: the PMU driver has been moved under
> drivers/perf/, which has been mentioned repeatedly by several people.
> This results in a different driver structure compared to my earlier
> version and the other two series, and I'd like to gather feedback and
> suggestions from the community on this approach. If the community would
> prefer to go with Lv Zheng’s version instead, I am perfectly fine with
> that as well.

We'd like to know if there's any comments related to putting IOMMU HPM
driver into drivers/perf folder and want to know if this is necessary.
We also hesitated if riscv iommu hpm should be put into drivers/perf,
and according to the followings, we finally decided to keep it in
drivers/iommu:
1. drivers/iommu has already a folder maintaining all IOMMU stuffs.
2. drivers/iommu has already defined HPM related registers.
3. Unlike PMCG of SMMU which is located in a separate namespace, RISC-V
    IOMMU only defines 5 registers in the same address space as its
    translation schemes.
4. For HPM schemes that is not a standalone device, Intel has put its
    perf driver under drivers/iommu/intel/perfmon.c.

Cheers,
Lv