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[34.142.211.105]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3acc963f36asm8219762a91.0.2026.10.19.01.24.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 19 Oct 2026 01:24:29 -0700 (PDT) Date: Mon, 19 Oct 2026 08:24:21 +0000 From: Pranjal Shrivastava To: Will Deacon Cc: iommu@lists.linux.dev, Joerg Roedel , Robin Murphy , Jason Gunthorpe , Mostafa Saleh , Nicolin Chen , Daniel Mentz , Ashish Mhetre , linux-arm-kernel@lists.infradead.org, Thomas Gleixner , Radu Rendec , Bjorn Helgaas , linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org, Greg Kroah-Hartman , rafael@kernel.org, Danilo Krummrich , driver-core@lists.linux.dev Subject: Re: [PATCH v11 11/16] iommu/arm-smmu-v3: Add CMDQ_PROD_STOP_FLAG to gate CMDQ submissions Message-ID: References: <20260929034510.2023173-1-praan@google.com> <20260929034510.2023173-12-praan@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Wed, Oct 07, 2026 at 03:08:01PM +0100, Will Deacon wrote: > On Tue, Sep 29, 2026 at 03:45:05AM +0000, Pranjal Shrivastava wrote: > > Introduce a new bit flag, CMDQ_PROD_STOP_FLAG (bit 30), in the command > > queue's producer index to safely gate command submissions during device > > suspension. > > > > The flag embeds the suspend state directly into the existing global state > > The flag is checked in the cmpxchg loop in arm_smmu_cmdq_issue_cmdlist(), > > which acts as a Point of Commitment, ensuring that no indices are > > reserved or committed once the SMMU begins suspending. > > I'm not familiar with the term "Point of Commitment". Is that something > you came up with? Without a general definition, I think I'd prefer to > avoid using it, especially as it sounds like something to do with the > Arm memory system (where we already have architectural terms such as > "Point of Coherency". > > > This prevents a situation of "abandoned batches" where indices are > > incremented but commands are never written, which would otherwise > > lead to timeout during the drain poll. > > > > Update queue_inc_prod_n() to preserve this flag during index > > calculations, ensuring that any in-flight commands that successfully > > passed the point of commitment can proceed to completion while the > > flag remains set. > > > > Suggested-by: Daniel Mentz > > Reviewed-by: Nicolin Chen > > Signed-off-by: Pranjal Shrivastava > > --- > > drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 35 +++++++++++++++++++-- > > drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h | 5 +++ > > 2 files changed, 38 insertions(+), 2 deletions(-) > > > > diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c > > index a123810fac57..7f89e2306942 100644 > > --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c > > +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c > > @@ -218,7 +218,8 @@ static int queue_sync_prod_in(struct arm_smmu_queue *q) > > static u32 queue_inc_prod_n(struct arm_smmu_ll_queue *q, int n) > > { > > u32 prod = Q_POS(q, q->prod) + n; > > - return Q_OVF(q->prod) | Q_POS(q, prod); > > + > > + return Q_OVF(q->prod) | Q_STOP(q->prod) | Q_POS(q, prod); > > } > > > > static void queue_poll_init(struct arm_smmu_device *smmu, > > @@ -726,13 +727,42 @@ int __arm_smmu_cmdq_issue_cmdlist(struct arm_smmu_device *smmu, > > do { > > u64 old; > > > > + /* > > + * If the SMMU is suspended/suspending, any new CMDs are elided. > > + * This loop is the Point of Commitment. If we haven't cmpxchg'd > > + * our new indices yet, we can safely bail. Once the indices are > > + * committed, we MUST write valid commands to those slots to > > + * avoid indefinite polling in the drain function. > > + */ > > + if (Q_STOP(llq.prod)) { > > + local_irq_restore(flags); > > + return 0; > > + } > > + > > while (!queue_has_space(&llq, n + sync)) { > > local_irq_restore(flags); > > + > > + /* Avoid waiting for space if the SMMU is suspending */ > > + if (Q_STOP(READ_ONCE(cmdq->q.llq.prod))) > > + return 0; > > + > > if (arm_smmu_cmdq_poll_until_not_full(smmu, cmdq, &llq)) > > dev_err_ratelimited(smmu->dev, "CMDQ timeout\n"); > > It's a bit grotty to have the extra READ_ONCE() here. Why not make > arm_smmu_cmdq_poll_until_not_full() return if the stop bit is set? > Sure, it already refreshes llq on every iteration, so I'll make it bail out on Q_STOP(llq->prod) and have the caller loop on !Q_STOP(llq.prod) && !queue_has_space(), leaving a single STOP check right before the cmpxchg. > > diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h > > index deefb17e31eb..794b258550dd 100644 > > --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h > > +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h > > @@ -394,6 +394,11 @@ static inline unsigned int arm_smmu_cdtab_l2_idx(unsigned int ssid) > > #define CMDQ_ERR_CERROR_ATC_INV_IDX 3 > > > > #define CMDQ_PROD_OWNED_FLAG Q_OVERFLOW_FLAG > > +#define CMDQ_PROD_STOP_FLAG (1U << 30) > > I'm unsure about this. Bit 30 of the cmdq prod register is RES0, so it's > plausible that the architecture allocates that bit for something in future. > For example, if the index field was extended, then I think that would > cause problems for the driver. > > Do we have to put the stop flag in the producer value, or can it just be > a flag elsewhere in memory? > Fair point, but the flag is SW-only, i.e. it's always stripped via CMDQ_PROD_IDX_MASK before anything reaches the HW and only ever tested via Q_STOP(). It's the same deal as CMDQ_PROD_OWNED_FLAG, which already uses bit 31 (also RES0 in SMMU_CMDQ_PROD, IHI0070 section 6.3.27) as a SW-only marker in llq.prod. Keeping it in prod is what makes the check + slot reservation atomic in the cmpxchg.. with a separate flag, a CPU could check it, then suspend sets it and samples the target, and the CPU would still reserve slots beyond that target. Thanks, Praan