[U-Boot] [RESEND PATCH 2/3] arm: introduce _relaxed MMIO accessors
Andre Przywara
andre.przywara at arm.com
Sun Feb 10 16:17:25 UTC 2019
The normal MMIO accessor macros (readX/writeX) guarantee a strong ordering,
even with normal memory accesses: https://lwn.net/Articles/698014/
For some MMIO operations (framebuffers being a prominent example) this is
not needed, and the rather costly barrier inserted on weakly ordered
systems like ARM costs some performance.
To mitigate this issue, Linux introduced readX_relaxed and
writeX_relaxed primitives, which only guarantee ordering between each
other, so are typically faster due to the missing barrier.
We probably do not care so much about performance in U-Boot, but want to
have those primitives for two other reasons:
- Being able to use the _relaxed macros simplifies porting drivers from
Linux.
- The missing barrier typically allows to generate smaller code, which can
relieve some chronically tight SPL builds.
Add those macros definitions by using the __raw versions of the
accessors, which matches an earlier (and less complicated) version of
the Linux implementation.
Signed-off-by: Andre Przywara <andre.przywara at arm.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
[On my experimental RK3399 after modifying a few drivers:]
Tested-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
---
arch/arm/include/asm/io.h | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/arch/arm/include/asm/io.h b/arch/arm/include/asm/io.h
index bcbaf0d83c..5b24b88efc 100644
--- a/arch/arm/include/asm/io.h
+++ b/arch/arm/include/asm/io.h
@@ -98,11 +98,21 @@ static inline void __raw_readsl(unsigned long addr, void *data, int longlen)
#define writel(v,c) ({ u32 __v = v; __iowmb(); __arch_putl(__v,c); __v; })
#define writeq(v,c) ({ u64 __v = v; __iowmb(); __arch_putq(__v,c); __v; })
+#define writeb_relaxed(v,c) __raw_writeb(v, c)
+#define writew_relaxed(v,c) __raw_writew(v, c)
+#define writel_relaxed(v,c) __raw_writel(v, c)
+#define writeq_relaxed(v,c) __raw_writeq(v, c)
+
#define readb(c) ({ u8 __v = __arch_getb(c); __iormb(); __v; })
#define readw(c) ({ u16 __v = __arch_getw(c); __iormb(); __v; })
#define readl(c) ({ u32 __v = __arch_getl(c); __iormb(); __v; })
#define readq(c) ({ u64 __v = __arch_getq(c); __iormb(); __v; })
+#define readb_relaxed(c) __raw_readb(c)
+#define readw_relaxed(c) __raw_readw(c)
+#define readl_relaxed(c) __raw_readl(c)
+#define readq_relaxed(c) __raw_readq(c)
+
/*
* The compiler seems to be incapable of optimising constants
* properly. Spell it out to the compiler in some cases.
--
2.14.5
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