linux-kernel - Re: [PATCH] swiotlb: iommu/dma: Move trace_swiotlb_bounced() into swiotlb_tbl_map

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Message-ID: <e2d856b7-0214-49ab-a4c5-4e6ee2541c6d@arm.com>
Date: Wed, 11 Jun 2025 15:28:52 +0100
From: Robin Murphy <robin.murphy@....com>
To: Steven Rostedt <rostedt@...dmis.org>, LKML
 <linux-kernel@...r.kernel.org>,
 Linux trace kernel <linux-trace-kernel@...r.kernel.org>,
 iommu@...ts.linux.dev
Cc: Christoph Hellwig <hch@...radead.org>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>
Subject: Re: [PATCH] swiotlb: iommu/dma: Move trace_swiotlb_bounced() into
 swiotlb_tbl_map_single()

On 2025-06-11 3:01 pm, Steven Rostedt wrote:
> From: Steven Rostedt <rostedt@...dmis.org>
> 
> I'm working on code that will warn when a tracepoint is defined but not
> used. As the TRACE_EVENT() logic still creates all the code regardless if
> something calls the trace_<event>() function. It wastes around 5K per trace
> event (less for tracepoints).
> 
> But it seems that the code in drivers/iommu/dma-iommu.c does the opposite.
> It calls the trace_swiotlb_bounced() tracepoint without it being defined.
> The tracepoint is defined in kernel/dma/swiotlb.c when CONFIG_SWIOTLB is
> defined, but this code exists when that config is not defined.
> 
> This now fails with my work because I have all the callers reference the
> tracepoint that they will call.
> 
> Thanks to the kernel test robot, it found this:
> 
>    https://lore.kernel.org/all/202506091015.7zd87kI7-lkp@intel.com/
> 
> Instead of calling trace_swiotlb_bounced() from drivers/iommu/dma-iommu.c
> where it is useless when CONFIG_SWIOTLB is not defined, move the
> tracepoint into swiotlb_tbl_map_single(). This also makes it consistent
> with which memory is being traced (physical as supposed to dma address).

Consistent with what? Certainly not the other callers which invoke the 
tracepoint with phys_to_dma(orig_addr), but will now do so twice with 
potentially different values. Arguably iommu-dma is already consistent 
as things stand, since it does not support static address offsets 
underneath IOMMU translation, and therefore orig_addr will always be 
equal to phys_to_dma(orig_addr) anyway. The point of interest really is 
in those other cases, where if there *were* a static DMA offset then 
phys_to_dma(orig_addr) would actually be a pretty meaningless value 
which is not used anywhere else and therefore not overly helpful to 
trace - neither the recognisable PA of the underlying memory itself, nor 
the actual DMA address which will be used subsequently, since at this 
point that's yet to be allocated.

> Fixes: ed18a46262be4 ("iommu/dma: Factor out a iommu_dma_map_swiotlb helper")

It's a fair bit older than that, try a63c357b9fd5 ("iommu/dma: Trace 
bounce buffer usage when mapping buffers") - looks like it's always just 
been hopeful of being DCE'd.

Thanks,
Robin.

> Closes: https://lore.kernel.org/all/20250610202457.5a599336@gandalf.local.home/
> Suggested-by: Christoph Hellwig <hch@...radead.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>   drivers/iommu/dma-iommu.c | 2 --
>   kernel/dma/swiotlb.c      | 2 ++
>   2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index ea2ef53bd4fe..1935b360cc94 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -1153,8 +1153,6 @@ static phys_addr_t iommu_dma_map_swiotlb(struct device *dev, phys_addr_t phys,
>   		return (phys_addr_t)DMA_MAPPING_ERROR;
>   	}
>   
> -	trace_swiotlb_bounced(dev, phys, size);
> -
>   	phys = swiotlb_tbl_map_single(dev, phys, size, iova_mask(iovad), dir,
>   			attrs);
>   
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index abcf3fa63a56..c112f1d98861 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -1379,6 +1379,8 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
>   	phys_addr_t tlb_addr;
>   	unsigned short pad_slots;
>   
> +	trace_swiotlb_bounced(dev, orig_addr, mapping_size);
> +
>   	if (!mem || !mem->nslabs) {
>   		dev_warn_ratelimited(dev,
>   			"Can not allocate SWIOTLB buffer earlier and can't now provide you with the DMA bounce buffer");