[U-Boot] [PATCH] Enable port-mapped access to 16550 UART

[Detlev Zundel <dzu@denx.de>]

Hi Graeme,


[...]

>>> +#ifdef CONFIG_X86
>>> +#define uart_writeb(x,y)     outb(x,(ulong)y)
>>> +#define uart_readb(y)                inb((ulong)y)
>>> +#else
>>> +#define uart_writeb(x,y) writeb(x,y)
>>> +#define uart_readb(y) readb(y)
>>> +#endif
>>
>> Why do you need a specific variant for X86 instead of implementing
>> writeb and readb correctly in the first place?
>
> For x86 readb and writeb provide volatile accessors to memory - These are
> used for memory-mapped devices (i.e. devices which are attached directly
> to the memory bus such as PCI devices etc). inb and outb provide access to
> I/O Ports. For example:
>
> writeb(0x12, 0x00001000) will generate something like:
>     movb $0x12, al
>     movl $0x00001000, ebx
>     movb al, ebx
>
> outb(0x12, 0x00001000) will generate something like:
>     movb $0x12, al
>     movl $0x00001000, ebx
>     outb al, ebx
>
> Looking at include/asm/asm-ppc/io.h it seems to me that, for PPC, there is
> no differentiation between readb/writeb and inb/outb other than that the
> user may define an optional IOBASE for inb/outb which shifts where in
> memory inb/outb accesses, but they are still memory accesses. So, for PPC,
> if IOBASE is 0, the above two examples will compile to identical code.
>
> (Having a look at the other arches, it appears that x86 is very unique in
> that inb/outb do not access memory)

Ok, I remember this icky in/out stuff from x86 now that you come to
mention it. 

So it seems we have to keep the distinction somehow.  Looking at
drivers/serial/8250.c to see what Linux does, we could start including
something like the "port.iotype" layer from there, although I feel this
is somewhat too heavy currently.  So in the meantime, I'd suggest that
we at least start using the Linux convention and turn all the register
accesses into "serial_{in,out}" and define these for X86 and !X86 like
you did.

This way should be somewhat clearer than defining a "writeb" not to be a
writeb after all, which I find confusing.

What do you think?

Cheers
  Detlev

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