Subject: Re: [PATCH 2/9] iommu/arm-smmu-v3: Use the HW arm_smmu_cmd in cmdq selection functions

From: Jason Gunthorpe <[email-masked]>
Date: Fri, 8 May 2026 12:54:32 -0300

On Fri, May 08, 2026 at 07:47:45AM +0000, Pranjal Shrivastava wrote:
> On Fri, May 01, 2026 at 11:29:11AM -0300, Jason Gunthorpe wrote:
> > Start removing the use of struct arm_smmu_cmdq_ent, starting at the
> > lower levels of the call chain. Change the functions that determine what
> > cmdq to issue the batch to into using struct arm_smmu_cmd directly.
> > 
> > Signed-off-by: Jason Gunthorpe <[email-masked]>
> > ---
> >  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c   | 47 ++++++++++++-------
> >  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h   |  8 ++--
> >  .../iommu/arm/arm-smmu-v3/tegra241-cmdqv.c    |  8 ++--
> >  3 files changed, 38 insertions(+), 25 deletions(-)
> 
> [...]
> 
> >  	index = cmds->num * CMDQ_ENT_DWORDS;
> > -	if (unlikely(arm_smmu_cmdq_build_cmd(&cmds->cmds[index], cmd))) {
> > -		dev_warn(smmu->dev, "ignoring unknown CMDQ opcode 0x%x\n",
> > -			 cmd->opcode);
> > -		return;
> > -	}
> > -
> > +	memcpy(&cmds->cmds[index], cmd.data, sizeof(cmd.data));
> >  	cmds->num++;
> >  }
> 
> The memcpy is a temporary step and goes way in Patch 4.

To be clear patch 4 still has the memcpy:

-	index = cmds->num * CMDQ_ENT_DWORDS;
-	memcpy(&cmds->cmds[index], cmd.data, sizeof(cmd.data));
-	cmds->num++;
+	cmds->cmds[cmds->num++] = cmd;
                           ^^^^^^^^^^^

The compiler just builds it.

The flow evolution is:

Now:
 struct arm_smmu_cmdq_ent (stack) -> arm_smmu_cmdq_build_cmd (batch) -> memcpy (queue)
Here:
 struct arm_smmu_cmdq_ent (stack) -> arm_smmu_cmdq_build_cmd (stack) -> memcpy (batch) -> memcpy (queue)
Done:
 struct arm_smmu_cmd (stack) -> memcpy (batch) -> memcpy (queue)

So the net result is the same before/after this series, we just carry
an extra stack copy till the last patch.

Thanks,
Jason