[PATCH, RX] Add target rx-*-linux

[Oleg Endo]

Hi,

On Tue, 2018-07-31 at 10:39 +0900, Yoshinori Sato wrote:
>Â 
> It standrad linux kernel.
> 
> My target is RX62N + 16MByte external memory.
> I think that connecting with a 32 bit bus is not so slow.

The latest RX71/RX65 support 60 MHz 32 bit data external SDRAM. Â On top
of that, there is no instruction/data cache for external SDRAM. Â The
system was not designed to operate like that. Â The primary use case for
RX is to run code from on-chip flash and use on-chip RAM as application
data. Â Every individual access to external memory by the CPU costs
about 3 bus cycles, from what I can see in the hardware manual...

> Also, since the code size is small, I think the influence of memory
> speed is relaxed.

Even if all the Linux + user application code fits into the on-chip
flash ROM, the problem will be the CPU accessing data in external
memory without cache. Â That's why I don't think it's practical to run
Linux on RX MCUs.  It will run, but  there are better embedded Linux
options for the same or lower price as RX MCUs. Â So I'm not sure how
useful this will be?

> 
> I think that it is 10% smaller than other CPUs.
> yo-satoh@yo-satoh-debian:~/linux (rx)$ rx-unknown-linux-size vmlinux
>    text	   data	    bss	    dec	    hex	
> filename
> Â 801156	Â Â 68556	Â 164568	1034280	Â Â fc828	
> vmlinux

My observation similar. RX ISA produces slightly smaller code than
other ISAs. Â But not that much smaller to compensate the slow external
memory :)

Just for my understanding, why is it needed to create a new ELF type
for that? Â Why can't rx-elf be used as-is?

Cheers,
Oleg