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

[Graeme Russ]

On Fri, Oct 30, 2009 at 2:42 AM, Detlev Zundel <dzu at denx.de> wrote:
> 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

Yes, I just had a look now - Although very elegant, it is very heavy

> 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?
>

So essentially go with my patch but rename uart_writeb to serial_out and
uart_readb to serial_in? If so, I'll re-spin

> Cheers
>  Detlev

Regards,

Graeme