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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; >
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