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On 01/18/2011 08:56 PM, Arnd Bergmann wrote:
> On Tuesday 18 January 2011 20:01:12 Lars-Peter Clausen wrote:
>> Well, i've though about that as well, but in the current asm-generic/io.h readl is
>> unconditionally defined as cpu_to_le32(__raw_readl(addr)) and ioread32 is defined as
>> readl.
>>
>> So unless an arch io.h undefines those macros and redefines them (which none of the
>> current archs does, as far as i can see), we are o
>>
>> If an arch chooses to redefine ioread or readl, it should probably also redefine
>> ioread{16,32}be.
>
> Right, but the header file also serves as a template for new architectures
> that cannot directly use it. I would prefer not to give a possibly bad example
> here, especially when it's in a rarely used function.
Maybe I'm missing something here, but if I have a big-endian architecture isn't
ioread{16,32}be what I should use to access iomapped memory?
>>> The right solution is probably to use swab16/swab32 for the
>>> big-endian functions. This also corrects the iowrite functions
>>> which really should be using cpu_to_be32 instead of be32_to_cpu
>>> (although they are always defined to be the same afaict.
>>
>> This would first cause a conversion to little-endian, which is a swap() in the
>> generic case and then you would call swap() again on the result. Which is basically a
>> noop, but I'm not sure if compilers will detect this.
>
> The overhead of the swab() is certainly dwarfed by the long time spent in
> readl().
Well at least the code size overhead is fundamental:
I have this simple function:
cycles_t get_cycles(void)
{
return ioread32be(CSR_TIMER_COUNTER(timer));
}
which when compiled for the lm32 arch results in the following assembler code
with #define ioread32be(addr) be32_to_cpu(__raw_readl(addr)):
00000128 <get_cycles>:
mvhi r2,0x4021
ori r2,r2,0xa100
lw r1,(r2+0)
lw r1,(r1+0)
ret
with #define ioread32be(addr) swap32(ioread32(addr)):
4001a694 <get_cycles>:
addi sp,sp,-16
sw (sp+16),r11
sw (sp+12),r12
sw (sp+8),r13
sw (sp+4),ra
mvhi r2,0x4021
ori r2,r2,0xa100
lw r1,(r2+0)
mvi r2,24
mvhi r13,0xff
lw r12,(r1+0)
mv r1,r12
calli 400f6f9c <__lshrsi3>
mv r11,r1
mvi r2,24
mv r1,r12
calli 400f6f6c <__ashlsi3>
or r11,r11,r1
mvi r2,8
andi r1,r12,0xff00
calli 400f6f6c <__ashlsi3>
or r11,r11,r1
mvi r2,8
and r1,r12,r13
calli 400f6f9c <__lshrsi3>
or r11,r11,r1
mv r1,r11
mvi r2,24
calli 400f6f9c <__lshrsi3>
mv r12,r1
mvi r2,24
mv r1,r11
calli 400f6f6c <__ashlsi3>
or r12,r12,r1
mvi r2,8
andi r1,r11,0xff00
calli 400f6f6c <__ashlsi3>
or r12,r12,r1
mvi r2,8
and r1,r11,r13
calli 400f6f9c <__lshrsi3>
or r1,r12,r1
lw ra,(sp+4)
lw r11,(sp+16)
lw r12,(sp+12)
lw r13,(sp+8)
addi sp,sp,16
ret
So I as someone who implements arch support has two options either redefine
ioread32be in the arch io header, or use __raw_readl everywhere to access iomap memory.
>
> I would prefer to swap twice in this case and let the compiler work it out
> if possible. The next best alternative would probably be to define both
> ioread and ioread_be using __raw_readl in combination with a le32_to_cpu
> or be32_to_cpu.
>
> Arnd
- - Lars
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