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

[Yoshinori Sato <ysato at users dot sourceforge dot jp>]

On Tue, 31 Jul 2018 21:49:37 +0900,
Oleg Endo wrote:
> 
> 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...

I agree.

Because I have not done benchmarks, I only have my own sense,
but it works faster than I suppose.

> > 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?

Although it is not ready yet, it is possible to place the code in the ROM area
including the user program.
I think that it is possible to create an environment where Linux can operate on
a single chip as long as it is an MCU with a large capacity memory.

> > 
> > 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?

Section name etc is different from general ELF.
standard ELF | rx-elf
.text        | P
.data        | D
.rodata      | C
.bss         | B

> Cheers,
> Oleg

-- 
Yoshinori Sato
<ysato@users.sourceforge.jp>