linux-kernel - Re: [PATCH v2] dma-mapping: add unlikely hint for error path in dma_mapping

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Date:   Mon, 14 Dec 2020 13:01:25 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Heiner Kallweit <hkallweit1@...il.com>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Barry Song <song.bao.hua@...ilicon.com>
Cc:     "open list:AMD IOMMU (AMD-VI)" <iommu@...ts.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] dma-mapping: add unlikely hint for error path in
 dma_mapping_error

On 2020-12-13 16:32, Heiner Kallweit wrote:
> Zillions of drivers use the unlikely() hint when checking the result of
> dma_mapping_error(). This is an inline function anyway, so we can move
> the hint into this function and remove it from drivers.

Reviewed-by: Robin Murphy <robin.murphy@....com>

FWIW I consider this case similar to the same hint in WARN_ON() and 
friends - it's a pretty severe condition that should never be expected 
to be hit in normal operation, so it's entirely logical for it to be 
implicitly unlikely. I struggle to imagine any case that would 
specifically *not* want that (or worse, want to hint it as likely). Some 
DMA API backends may spend considerable time trying as hard as possible 
to make a mapping work before eventually admitting defeat, so the idea 
of ever trying to optimise at the driver level for failure in hot paths 
just makes no sense.

Thanks,
Robin.

> Signed-off-by: Heiner Kallweit <hkallweit1@...il.com>
> ---
> v2:
> Split the big patch into the change for dma-mapping.h and follow-up
> patches per subsystem that will go through the trees of the respective
> maintainers.
> ---
>   include/linux/dma-mapping.h | 2 +-
>   kernel/dma/map_benchmark.c  | 2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 2e49996a8..6177e20b5 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -95,7 +95,7 @@ static inline int dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
>   {
>   	debug_dma_mapping_error(dev, dma_addr);
>   
> -	if (dma_addr == DMA_MAPPING_ERROR)
> +	if (unlikely(dma_addr == DMA_MAPPING_ERROR))
>   		return -ENOMEM;
>   	return 0;
>   }
> diff --git a/kernel/dma/map_benchmark.c b/kernel/dma/map_benchmark.c
> index b1496e744..901420a5d 100644
> --- a/kernel/dma/map_benchmark.c
> +++ b/kernel/dma/map_benchmark.c
> @@ -78,7 +78,7 @@ static int map_benchmark_thread(void *data)
>   
>   		map_stime = ktime_get();
>   		dma_addr = dma_map_single(map->dev, buf, PAGE_SIZE, map->dir);
> -		if (unlikely(dma_mapping_error(map->dev, dma_addr))) {
> +		if (dma_mapping_error(map->dev, dma_addr)) {
>   			pr_err("dma_map_single failed on %s\n",
>   				dev_name(map->dev));
>   			ret = -ENOMEM;
>