Re: Question About __REG32 marco

["colorant" <colorant@163.com>]

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Hi Dave

> On Tuesday 20 November 2007, colorant wrote:
>> 
>>     1:
>>     Why this macro need to be defined in this way ?  Why borther to use an array ?
>>     Can't we just use something like :
>> 
>>     *(volatile u32 *)(vaddr&~0x3) to get the value from vaddr with word align ?
> 
> It generalizes __REG2() from arch-pxa/hardware.h ... where,
> as the comment notes, GCC would otherwise be incapable of
> generating decent code.  It's possible GCC code generation
> has gotten smarter since then.
> 
> Note that such macros are supposed to be used only with
> constant addresses, so code can treat registers as if they
> were global variables.  Some people dislike that style;
> others find it more understandable than __raw_writel()
> style accessors.
> 
> When it works right, driver code will have a register with
> the base address of a controller, and all accesses to that
> controller will use single word load/store instructions which
> embed the offsets of the various registers being accessed
> against that base register.
> 
> The main alternative addressing styles were notably larger
> (I recall observing 1-2 KBytes per driver) and slower.
> 

Yes, I can see that from the obj-code. Thanks.

> 
>>     2:
>>     Why this offset array need to be 4096 in size ? shouldn't it be 1024 ?
>>     since it already point to a u32 value. and the >>2 make the macro never
>>     got chance to reach a index great than 1024 !  
> 
> Look at the ARM instructions that are used.  The address
> range may be 4KBytes, but the values are 4 bytes each.
> So the range of their indices is just 1K.  (As implied
> by the comments adjacent to those macro definitions, which
> point out the use of LDR/STR for 8 and 32 bit values...)
> 

This is what I am puzzled, since the index is just 1K, the following definition :

typedef struct { volatile u32 offset[4096]; } __regbase32;

shouldn't be:

 typedef struct { volatile u32 offset[1024]; } __regbase32; ?

Raymond


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