From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 8211241E6DC; Tue, 13 Oct 2026 11:35:45 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791891347; cv=none; b=hR2UUN6JuLLv9Iv6zSx/EqlNGPKKlAg6Fw/8onRWnuPdKwu9vKOOQ6/p8UYbvNeHABQMVgRuzijYorZezhw2AFVzUrP96ZUgkiZzu1XJ+ljgXs9sRDBS8kB1wq2OqhR6iR/7MugCnTA8cbzjD7qV+onOHDfXgJmLvTt+dak0eg8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791891347; c=relaxed/simple; bh=iFHnFsr3NjAcsRUa/CVUBKDHfKdX/CfcLbNrrMoPACE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Ff/LehhZq6XfTwzj9qfyozU/e04tV7yzB7J4SU2XxvdRKQu8sZbXpmoZMtk/k8CDi43rLAGYGhPN7mqcfmhqNRDfWl48aL7n0yjwYTcDDb5MZLu8nHCgs8eOgxXnlKdIc8686P/uKDBlr4YxqhOwJTNTtzHZilQw4B0J9MAXgRQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=Gl4+NGju; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="Gl4+NGju" Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 977A01570; Tue, 13 Oct 2026 04:35:41 -0700 (PDT) Received: from [10.2.212.23] (e121345-lin.cambridge.arm.com [10.2.212.23]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A23143F66F; Tue, 13 Oct 2026 04:35:42 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1791891344; bh=iFHnFsr3NjAcsRUa/CVUBKDHfKdX/CfcLbNrrMoPACE=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Gl4+NGjuoXJn53qe90hDjficciUU0vZiX+yyPYW+ZDx13uFc4Q5PLHcJhbPEDlp/W pzQnrSxhy37kEHopU8W//0TMy1atxB3VauY7j+UROpxd0jwNa1Cmp9OEaDoa+9UVbS qL35dEEYtZsulURYhEruyJQNPy9gWwgtw5g72G7A= Message-ID: <370a947c-601b-434c-befd-4896ed523ced@arm.com> Date: Tue, 13 Oct 2026 12:35:40 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v11 11/16] iommu/arm-smmu-v3: Add CMDQ_PROD_STOP_FLAG to gate CMDQ submissions To: Will Deacon , Pranjal Shrivastava Cc: 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 References: <20260929034510.2023173-1-praan@google.com> <20260929034510.2023173-12-praan@google.com> From: Robin Murphy Content-Language: en-GB In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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? Thanks, Robin.