Subject: Re: [PATCH v3 1/3] iommu/arm-smmu-v3: Factor out CMDQ batch force-sync conditions

From: Nicolin Chen <[email-masked]>
Date: Tue, 2 Jun 2026 13:08:26 -0700

On Tue, Jun 02, 2026 at 08:55:10PM +0100, Will Deacon wrote:
> On Mon, Jun 01, 2026 at 10:48:43AM +0000, Ashish Mhetre wrote:
> > From: Nicolin Chen <[email-masked]>
> > 
> > arm_smmu_cmdq_batch_add_cmd_p() carries two distinct reasons for
> > flushing the current batch with a CMD_SYNC before appending the
> > new command:
> > 
> >   - The batch's pre-assigned cmdq does not support the new command.
> >   - The Arm erratum 2812531 workaround (ARM_SMMU_OPT_CMDQ_FORCE_SYNC)
> >     forces a SYNC at one entry before the batch is full.
> > 
> > Lift those checks into a new arm_smmu_cmdq_batch_force_sync() helper
> > so that adding another force-sync condition becomes a one-line
> > addition. No functional change.
> > 
> > Signed-off-by: Nicolin Chen <[email-masked]>
> > Signed-off-by: Ashish Mhetre <[email-masked]>
> > ---
> >  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 26 ++++++++++++++++-----
> >  1 file changed, 20 insertions(+), 6 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 9be589d14a3b..4d29bd343460 100644
> > --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > @@ -847,16 +847,30 @@ static void arm_smmu_cmdq_batch_init_cmd(struct arm_smmu_device *smmu,
> >  	cmds->cmdq = arm_smmu_get_cmdq(smmu, cmd);
> >  }
> >  
> > +static bool arm_smmu_cmdq_batch_force_sync(struct arm_smmu_device *smmu,
> > +					   struct arm_smmu_cmdq_batch *cmds,
> > +					   struct arm_smmu_cmd *cmd)
> > +{
> > +	if (!cmds->num)
> > +		return false;
> 
> This check seems new?

You are right. Maybe the commit message should have mentioned that
there is a slight behavior change to the unsupported_cmd path:

 - Before: if cmds->num = 0 && unsupported_cmd, it would issue an
           empty batch (one CMD_SYNC)
 - After : if cmds->num = 0, no issue on the empty batch

With that, I think it's good to have?

Thanks
Nicolin