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Subject: Re: [PATCH v2] dma-mapping: add unlikely hint for error path in
 dma_mapping_error
To:     Robin Murphy <robin.murphy@arm.com>,
        Christoph Hellwig <hch@lst.de>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Barry Song <song.bao.hua@hisilicon.com>
Cc:     "open list:AMD IOMMU (AMD-VI)" <iommu@lists.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <39581f9a-0066-ad98-094d-d41082145e23@gmail.com>
 <ba14be47-6c77-94d6-9904-b4679e62091b@arm.com>
From:   Heiner Kallweit <hkallweit1@gmail.com>
Message-ID: <9cd308a1-2222-8716-2002-c0dacfaa6d91@gmail.com>
Date:   Fri, 8 Jan 2021 17:45:05 +0100
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On 14.12.2020 14:01, Robin Murphy wrote:
> 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@arm.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@gmail.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;
>>

Is this patch going to make it to linux-next?