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[34.142.211.105]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3acc4872861sm16015390a91.7.2026.10.19.01.28.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 19 Oct 2026 01:28:37 -0700 (PDT) Date: Mon, 19 Oct 2026 08:28:29 +0000 From: Pranjal Shrivastava To: Robin Murphy Cc: Will Deacon , iommu@lists.linux.dev, Joerg Roedel , 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> <370a947c-601b-434c-befd-4896ed523ced@arm.com> Precedence: bulk X-Mailing-List: linux-pci@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: <370a947c-601b-434c-befd-4896ed523ced@arm.com> On Tue, Oct 13, 2026 at 12:35:40PM +0100, Robin Murphy wrote: > On 07/10/2026 3:08 pm, 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? > > > > > 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? > > Logically, it feels like this should happen earlier in the process anyway - > if we're entering a state where we don't want to accept _any_ new command > submissions, then surely arm_smmu_get_cmdq() should just start saying "No, > there is no valid queue to submit this command." to everything? > We can do that as well, but it can only be an early-out.. the STOP_FLAG may not be set when arm_smmu_get_cmdq() looks at it, but get set right after, before we reserve our slots in arm_smmu_cmdq_issue_cmdlist(). So we'd still need the check at the point where we actually commit, i.e. the cmpxchg on llq.prod. Thanks, Praan