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

[Oleg Endo <oleg dot endo at t-online dot de>]

Hi,

On Tue, 2018-07-31 at 10:39 +0900, Yoshinori Sato wrote:
> 
> 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.  On top
of that, there is no instruction/data cache for external SDRAM.  The
system was not designed to operate like that.  The primary use case for
RX is to run code from on-chip flash and use on-chip RAM as application
data.  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.  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.  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
>  801156	  68556	 164568	1034280	  fc828	
> vmlinux

My observation similar. RX ISA produces slightly smaller code than
other ISAs.  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?  Why can't rx-elf be used as-is?

Cheers,
Oleg