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SubjectRe: [RFC] avoid indirect calls for DMA direct mappings
On Thu, Dec 06, 2018 at 10:28:22AM -0800, Linus Torvalds wrote:
> which is counterproductive because it checks for the fast case *after*
> you've done a load (and a check) that is entirely pointless for the
> fast case.
>
> Put another way, you made the fast case unnecessarily slow.
>
> If this fast case matters (and the whole point of the series is that
> it *does* matter), then you should make sure that
>
> (a) the dma_is_direct() function/macro uses "likely()" for the test

I'm a little worried about that. Yes, for the common case it is
likely. But if you run a setup where you say always have an iommu
it is not, in fact it is never called in that case, but we only know
that at runtime.

> (b) the code is organized like this instead:
>
> + if (dma_is_direct(ops))
> + dma_direct_unmap_page(dev, addr, size, dir, attrs);
> + else if (ops->unmap_page)
> + ops->unmap_page(dev, addr, size, dir, attrs);
>
> because now you avoid that unnecessary conditional for the common
> case, and you hopefully never even access the pointer (because you
> know what's behind it: the direct ops cases that you're
> short-circuiting).

Agreed on that, I've fixed it up now (current patch attached below).

> In fact, as a further micro-optimization it might be a good idea to
> just specify that the dma_is_direct() ops is a special pointer
> (perhaps even just say that "NULL means it's direct"), because that
> then makes the fast-case test much simpler (avoids a whole nasty
> constant load, and testing for NULL in particular is often much
> better).

So I wanted to do the NULL case, and it would be much nicer. But the
arm folks went to great length to make sure they don't have a default
set of dma ops and require it to be explicitly set on every device
to catch cases where people don't set things up properly and I didn't
want to piss them off.. But maybe I should just go for it and see who
screams, as the benefit is pretty obvious.

---
From 89cc8ab03798177339ad916efabd320e792ee034 Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <hch@lst.de>
Date: Mon, 3 Dec 2018 16:49:29 +0100
Subject: dma-mapping: bypass indirect calls for dma-direct

Avoid expensive indirect calls in the fast path DMA mapping
operations by directly calling the dma_direct_* ops if we are using
the directly mapped DMA operations.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
include/linux/dma-direct.h | 17 ------
include/linux/dma-mapping.h | 110 ++++++++++++++++++++++++++++++++----
kernel/dma/direct.c | 13 +++--
3 files changed, 106 insertions(+), 34 deletions(-)

diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h
index 3b0a3ea3876d..b7338702592a 100644
--- a/include/linux/dma-direct.h
+++ b/include/linux/dma-direct.h
@@ -60,22 +60,5 @@ void dma_direct_free_pages(struct device *dev, size_t size, void *cpu_addr,
struct page *__dma_direct_alloc_pages(struct device *dev, size_t size,
dma_addr_t *dma_handle, gfp_t gfp, unsigned long attrs);
void __dma_direct_free_pages(struct device *dev, size_t size, struct page *page);
-dma_addr_t dma_direct_map_page(struct device *dev, struct page *page,
- unsigned long offset, size_t size, enum dma_data_direction dir,
- unsigned long attrs);
-void dma_direct_unmap_page(struct device *dev, dma_addr_t addr,
- size_t size, enum dma_data_direction dir, unsigned long attrs);
-int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents,
- enum dma_data_direction dir, unsigned long attrs);
-void dma_direct_unmap_sg(struct device *dev, struct scatterlist *sgl,
- int nents, enum dma_data_direction dir, unsigned long attrs);
-void dma_direct_sync_single_for_device(struct device *dev,
- dma_addr_t addr, size_t size, enum dma_data_direction dir);
-void dma_direct_sync_sg_for_device(struct device *dev,
- struct scatterlist *sgl, int nents, enum dma_data_direction dir);
-void dma_direct_sync_single_for_cpu(struct device *dev,
- dma_addr_t addr, size_t size, enum dma_data_direction dir);
-void dma_direct_sync_sg_for_cpu(struct device *dev,
- struct scatterlist *sgl, int nents, enum dma_data_direction dir);
int dma_direct_supported(struct device *dev, u64 mask);
#endif /* _LINUX_DMA_DIRECT_H */
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index 8916499d2805..98e4dd67c906 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -221,6 +221,69 @@ static inline const struct dma_map_ops *get_dma_ops(struct device *dev)
}
#endif

+static inline bool dma_is_direct(const struct dma_map_ops *ops)
+{
+ return IS_ENABLED(CONFIG_DMA_DIRECT_OPS) && ops == &dma_direct_ops;
+}
+
+/*
+ * All the dma_direct_* declarations are here just for the indirect call bypass,
+ * and must not be used directly drivers!
+ */
+dma_addr_t dma_direct_map_page(struct device *dev, struct page *page,
+ unsigned long offset, size_t size, enum dma_data_direction dir,
+ unsigned long attrs);
+int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents,
+ enum dma_data_direction dir, unsigned long attrs);
+
+#if defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_DEVICE) || \
+ defined(CONFIG_SWIOTLB)
+void dma_direct_sync_single_for_device(struct device *dev,
+ dma_addr_t addr, size_t size, enum dma_data_direction dir);
+void dma_direct_sync_sg_for_device(struct device *dev,
+ struct scatterlist *sgl, int nents, enum dma_data_direction dir);
+#else
+static inline void dma_direct_sync_single_for_device(struct device *dev,
+ dma_addr_t addr, size_t size, enum dma_data_direction dir)
+{
+}
+static inline void dma_direct_sync_sg_for_device(struct device *dev,
+ struct scatterlist *sgl, int nents, enum dma_data_direction dir)
+{
+}
+#endif
+
+#if defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU) || \
+ defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU_ALL) || \
+ defined(CONFIG_SWIOTLB)
+void dma_direct_unmap_page(struct device *dev, dma_addr_t addr,
+ size_t size, enum dma_data_direction dir, unsigned long attrs);
+void dma_direct_unmap_sg(struct device *dev, struct scatterlist *sgl,
+ int nents, enum dma_data_direction dir, unsigned long attrs);
+void dma_direct_sync_single_for_cpu(struct device *dev,
+ dma_addr_t addr, size_t size, enum dma_data_direction dir);
+void dma_direct_sync_sg_for_cpu(struct device *dev,
+ struct scatterlist *sgl, int nents, enum dma_data_direction dir);
+#else
+static inline void dma_direct_unmap_page(struct device *dev, dma_addr_t addr,
+ size_t size, enum dma_data_direction dir, unsigned long attrs)
+{
+}
+static inline void dma_direct_unmap_sg(struct device *dev,
+ struct scatterlist *sgl, int nents, enum dma_data_direction dir,
+ unsigned long attrs)
+{
+}
+static inline void dma_direct_sync_single_for_cpu(struct device *dev,
+ dma_addr_t addr, size_t size, enum dma_data_direction dir)
+{
+}
+static inline void dma_direct_sync_sg_for_cpu(struct device *dev,
+ struct scatterlist *sgl, int nents, enum dma_data_direction dir)
+{
+}
+#endif
+
static inline dma_addr_t dma_map_single_attrs(struct device *dev, void *ptr,
size_t size,
enum dma_data_direction dir,
@@ -231,9 +294,12 @@ static inline dma_addr_t dma_map_single_attrs(struct device *dev, void *ptr,

BUG_ON(!valid_dma_direction(dir));
debug_dma_map_single(dev, ptr, size);
- addr = ops->map_page(dev, virt_to_page(ptr),
- offset_in_page(ptr), size,
- dir, attrs);
+ if (dma_is_direct(ops))
+ addr = dma_direct_map_page(dev, virt_to_page(ptr),
+ offset_in_page(ptr), size, dir, attrs);
+ else
+ addr = ops->map_page(dev, virt_to_page(ptr),
+ offset_in_page(ptr), size, dir, attrs);
debug_dma_map_page(dev, virt_to_page(ptr),
offset_in_page(ptr), size,
dir, addr, true);
@@ -248,7 +314,9 @@ static inline void dma_unmap_single_attrs(struct device *dev, dma_addr_t addr,
const struct dma_map_ops *ops = get_dma_ops(dev);

BUG_ON(!valid_dma_direction(dir));
- if (ops->unmap_page)
+ if (dma_is_direct(ops))
+ dma_direct_unmap_page(dev, addr, size, dir, attrs);
+ else if (ops->unmap_page)
ops->unmap_page(dev, addr, size, dir, attrs);
debug_dma_unmap_page(dev, addr, size, dir, true);
}
@@ -265,7 +333,10 @@ static inline int dma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
int ents;

BUG_ON(!valid_dma_direction(dir));
- ents = ops->map_sg(dev, sg, nents, dir, attrs);
+ if (dma_is_direct(ops))
+ ents = dma_direct_map_sg(dev, sg, nents, dir, attrs);
+ else
+ ents = ops->map_sg(dev, sg, nents, dir, attrs);
BUG_ON(ents < 0);
debug_dma_map_sg(dev, sg, nents, ents, dir);

@@ -280,7 +351,9 @@ static inline void dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sg

BUG_ON(!valid_dma_direction(dir));
debug_dma_unmap_sg(dev, sg, nents, dir);
- if (ops->unmap_sg)
+ if (dma_is_direct(ops))
+ dma_direct_unmap_sg(dev, sg, nents, dir, attrs);
+ else if (ops->unmap_sg)
ops->unmap_sg(dev, sg, nents, dir, attrs);
}

@@ -294,7 +367,10 @@ static inline dma_addr_t dma_map_page_attrs(struct device *dev,
dma_addr_t addr;

BUG_ON(!valid_dma_direction(dir));
- addr = ops->map_page(dev, page, offset, size, dir, attrs);
+ if (dma_is_direct(ops))
+ addr = dma_direct_map_page(dev, page, offset, size, dir, attrs);
+ else
+ addr = ops->map_page(dev, page, offset, size, dir, attrs);
debug_dma_map_page(dev, page, offset, size, dir, addr, false);

return addr;
@@ -308,7 +384,9 @@ static inline void dma_unmap_page_attrs(struct device *dev,
const struct dma_map_ops *ops = get_dma_ops(dev);

BUG_ON(!valid_dma_direction(dir));
- if (ops->unmap_page)
+ if (dma_is_direct(ops))
+ dma_direct_unmap_page(dev, addr, size, dir, attrs);
+ else if (ops->unmap_page)
ops->unmap_page(dev, addr, size, dir, attrs);
debug_dma_unmap_page(dev, addr, size, dir, false);
}
@@ -355,7 +433,9 @@ static inline void dma_sync_single_for_cpu(struct device *dev, dma_addr_t addr,
const struct dma_map_ops *ops = get_dma_ops(dev);

BUG_ON(!valid_dma_direction(dir));
- if (ops->sync_single_for_cpu)
+ if (dma_is_direct(ops))
+ dma_direct_sync_single_for_cpu(dev, addr, size, dir);
+ else if (ops->sync_single_for_cpu)
ops->sync_single_for_cpu(dev, addr, size, dir);
debug_dma_sync_single_for_cpu(dev, addr, size, dir);
}
@@ -374,7 +454,9 @@ static inline void dma_sync_single_for_device(struct device *dev,
const struct dma_map_ops *ops = get_dma_ops(dev);

BUG_ON(!valid_dma_direction(dir));
- if (ops->sync_single_for_device)
+ if (dma_is_direct(ops))
+ dma_direct_sync_single_for_device(dev, addr, size, dir);
+ else if (ops->sync_single_for_device)
ops->sync_single_for_device(dev, addr, size, dir);
debug_dma_sync_single_for_device(dev, addr, size, dir);
}
@@ -393,7 +475,9 @@ dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg,
const struct dma_map_ops *ops = get_dma_ops(dev);

BUG_ON(!valid_dma_direction(dir));
- if (ops->sync_sg_for_cpu)
+ if (dma_is_direct(ops))
+ dma_direct_sync_sg_for_cpu(dev, sg, nelems, dir);
+ else if (ops->sync_sg_for_cpu)
ops->sync_sg_for_cpu(dev, sg, nelems, dir);
debug_dma_sync_sg_for_cpu(dev, sg, nelems, dir);
}
@@ -405,7 +489,9 @@ dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,
const struct dma_map_ops *ops = get_dma_ops(dev);

BUG_ON(!valid_dma_direction(dir));
- if (ops->sync_sg_for_device)
+ if (dma_is_direct(ops))
+ dma_direct_sync_sg_for_device(dev, sg, nelems, dir);
+ else if (ops->sync_sg_for_device)
ops->sync_sg_for_device(dev, sg, nelems, dir);
debug_dma_sync_sg_for_device(dev, sg, nelems, dir);

diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 372c1955454a..cd535e7c67d7 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -223,6 +223,7 @@ void dma_direct_sync_single_for_device(struct device *dev,
if (!dev_is_dma_coherent(dev))
arch_sync_dma_for_device(dev, paddr, size, dir);
}
+EXPORT_SYMBOL(dma_direct_sync_single_for_device);

void dma_direct_sync_sg_for_device(struct device *dev,
struct scatterlist *sgl, int nents, enum dma_data_direction dir)
@@ -240,6 +241,7 @@ void dma_direct_sync_sg_for_device(struct device *dev,
dir);
}
}
+EXPORT_SYMBOL(dma_direct_sync_sg_for_device);
#endif

#if defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU) || \
@@ -258,6 +260,7 @@ void dma_direct_sync_single_for_cpu(struct device *dev,
if (is_swiotlb_buffer(paddr))
swiotlb_tbl_sync_single(dev, paddr, size, dir, SYNC_FOR_CPU);
}
+EXPORT_SYMBOL(dma_direct_sync_single_for_cpu);

void dma_direct_sync_sg_for_cpu(struct device *dev,
struct scatterlist *sgl, int nents, enum dma_data_direction dir)
@@ -277,6 +280,7 @@ void dma_direct_sync_sg_for_cpu(struct device *dev,
if (!dev_is_dma_coherent(dev))
arch_sync_dma_for_cpu_all(dev);
}
+EXPORT_SYMBOL(dma_direct_sync_sg_for_cpu);

void dma_direct_unmap_page(struct device *dev, dma_addr_t addr,
size_t size, enum dma_data_direction dir, unsigned long attrs)
@@ -289,6 +293,7 @@ void dma_direct_unmap_page(struct device *dev, dma_addr_t addr,
if (is_swiotlb_buffer(phys))
swiotlb_tbl_unmap_single(dev, phys, size, dir, attrs);
}
+EXPORT_SYMBOL(dma_direct_unmap_page);

void dma_direct_unmap_sg(struct device *dev, struct scatterlist *sgl,
int nents, enum dma_data_direction dir, unsigned long attrs)
@@ -300,11 +305,7 @@ void dma_direct_unmap_sg(struct device *dev, struct scatterlist *sgl,
dma_direct_unmap_page(dev, sg->dma_address, sg_dma_len(sg), dir,
attrs);
}
-#else
-void dma_direct_unmap_sg(struct device *dev, struct scatterlist *sgl,
- int nents, enum dma_data_direction dir, unsigned long attrs)
-{
-}
+EXPORT_SYMBOL(dma_direct_unmap_sg);
#endif

static inline bool dma_direct_possible(struct device *dev, dma_addr_t dma_addr,
@@ -331,6 +332,7 @@ dma_addr_t dma_direct_map_page(struct device *dev, struct page *page,
arch_sync_dma_for_device(dev, phys, size, dir);
return dma_addr;
}
+EXPORT_SYMBOL(dma_direct_map_page);

int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents,
enum dma_data_direction dir, unsigned long attrs)
@@ -352,6 +354,7 @@ int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents,
dma_direct_unmap_sg(dev, sgl, i, dir, attrs | DMA_ATTR_SKIP_CPU_SYNC);
return 0;
}
+EXPORT_SYMBOL(dma_direct_map_sg);

/*
* Because 32-bit DMA masks are so common we expect every architecture to be
--
2.19.1
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