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Subject: Re: [PATCH v2] dma-mapping: add unlikely hint for error path in
 dma_mapping_error
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>,
        Christoph Hellwig <hch@lst.de>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.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>
 <93ac85090ffa4313a7dc87a38d1f6aa1@hisilicon.com>
From:   Heiner Kallweit <hkallweit1@gmail.com>
Message-ID: <2faa40b9-e632-b9f3-6de0-471f0b7a6cae@gmail.com>
Date:   Sun, 13 Dec 2020 22:23:29 +0100
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Am 13.12.2020 um 22:27 schrieb Song Bao Hua (Barry Song):
> 
> 
>> -----Original Message-----
>> From: Heiner Kallweit [mailto:hkallweit1@gmail.com]
>> Sent: Monday, December 14, 2020 5:33 AM
>> To: Christoph Hellwig <hch@lst.de>; Marek Szyprowski
>> <m.szyprowski@samsung.com>; Robin Murphy <robin.murphy@arm.com>; Song Bao Hua
>> (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>
>> Subject: [PATCH v2] dma-mapping: add unlikely hint for error path in
>> dma_mapping_error
>>
>> 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.
>>
>> Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
> 
> not sure if this is really necessary. It seems the original code
> is more readable. Readers can more easily understand we are
> predicting the branch based on the return value of
> dma_mapping_error().
> 
I basically see two points promoting the proposed change:
1. Driver authors shouldn't have to think (as far as possible) about
   whether a branch prediction hint could make sense for a standard
   core API call. I saw quite some past discussions about when
   something is unlikely enough so that an unlikely() makes sense.
   If the core can hide some more complexity from drivers, then
   I think it's a good thing.
2. If we ever want or have to change the use of unlikely with
   dma_mapping_error(), then we have to do it in just one place.

> Anyway, I don't object to this one. if other people like it, I am
> also ok with it.
> 
>> ---
>> 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;
>> --
>> 2.29.2
> 
> Thanks
> Barry
>