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 <[email protected]>
---
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
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 <[email protected]>
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 <[email protected]>
> ---
> 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;
>
Am 13.12.2020 um 22:27 schrieb Song Bao Hua (Barry Song):
>
>
>> -----Original Message-----
>> From: Heiner Kallweit [mailto:[email protected]]
>> Sent: Monday, December 14, 2020 5:33 AM
>> To: Christoph Hellwig <[email protected]>; Marek Szyprowski
>> <[email protected]>; Robin Murphy <[email protected]>; Song Bao Hua
>> (Barry Song) <[email protected]>
>> Cc: open list:AMD IOMMU (AMD-VI) <[email protected]>; Linux
>> Kernel Mailing List <[email protected]>
>> 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 <[email protected]>
>
> 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
>
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 <[email protected]>
>
> 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 <[email protected]>
>> ---
>> 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?