Re: [linuxsh-dev] SH-5 integration issues

[David M. <dav...@st...>]

mr...@0x... wrote:
> 
> Paul and I have been trying to determine the best fit for the SH-5 port for
> 2.5.  The problem lies in the SH-5's (and SH-6's) addressing - it's 32-bit
> versus 64-bit, so the standard kernel convention for a 64-bit port doesn't
> necessarily qualify it as "sh64" (but hey, we're kernel hackers, we can
> bend the rules :P).

Well, I don't see any problem calling it a 64 bit. For me, the
fundamental criteria
for calling it a 64 bit port is that a pointer is 64 bits. This is
supported by the 
gcc toolchain (I've never tried it though), and all the internal
registers are quite 
happy with 64 bits. Yes, it is true that at the moment on the SH5 you
can't address more 
than 32 bits of physical, but I would view this as an implementation
detail, not an architecture 
restriction.  I agree that is a pain that the 64bitness is implemented
by sign extension
rather than real bits, but it is possible to build a kernel that would
work with this way 
of doing things and "real" 64 bits without too much pain I think.

Anyway, the current port isn't 64 bit, which is why it is an sh5
directory rather than
sh64. It should really be called shmedia in retrospect I suppose.

> Moving the SH-5 port in with the sh/ arch is a challenge

Absolutely. What I do not understand is what benefit you see from doing
it this way. 
As far as I can see it will simply make a mess of the current SH
support, as it is 
a different machine from an OS point of view.  Every arch specific file
will be duplicated, 
and will create an unholy mess for no benefit. Please explain what you
see as the benefits.


> is trying to plan for structuring future SH processors.  Will the SH-7 and
> SH-8 retain the SHmedia instructions and 32-bit addressing?  Or will
> Hitachi have a change of mind and switch to 64-bit addressing if the market
> calls for it?
> 

Rememeber that the architecture of future SH processors is controlled by
SuperH Inc,
not Hitachi. However, Hitachi and ourselves at ST can be safely
described as major customers 
of SuperH. I also think you have answered your own question, if somebody
wants a real 64 machine
I'm sure SuperH will be more than happy to oblige, given that the
architecture is already 64 bit 
so a 64 bit everywhere machine would be very easy for them to build. I
do not think they will
categorically state that they will never build a full 64 bit machine.
Anybody at superh care to 
comment?

As to SHmedia, I think that it is here to stay. It is the main
instructions set of 
the machine. A more pertinant question would be how long  SHcompact
stays around for.
Backward compatibilty costs both in terms of silicon area and
verification, so it is 
not beyond the realms of possibility that SuperH would produce a future
processor that 
is SHmedia only.  I can't speak for SuperH here, but it must be regarded
as a possibility.
Again I suspect that you will not get a statement saying SuperH will
always support
SHcompact in hardware indefinately for all future machines. Comments
from SuperH are invited.


> If future SH processors are "true" 64-bit machines (well addressing at
> least), then it makes sense for the SH-5 and SH-6 to live with the SH-3 and
> SH-4 code, but if the SH-7 and SH-8 retain current SHmedia and 32-bit
> addressing (e.g. they won't differ "externally" from the SH-5 or SH-6), then
> it makes sense for all SHmedia capable processors to live in "sh64".  This is
> why we need a bit of clarification (if at all possible) of the plans to stick
> with 32-bit addressing, so it'll potentially save us some work down the road.
> 

Again, I don't understand why you want to include sh5 code in
with the SH3/4 stuff. It will be a maintenance nightmare. If we think a
64 bit port
is a good idea, then lets create an sh64 directory and be done with it.
Otherwise lets 
stick to sh5 as a new 32 architecture (since it is effectively a new
machine). 

> Is this making sense to people?  We need your insights and suggestions
> before we can make the formal proposal of SH-5 inclusion :).
> 

Sort of, but I don't agree with you:-) I don't see the benefits, and I
can see the problems.

And also, I confess I am confused to exactly who you are going to make
this formal proposal to. Could you explain this further?

Cheers!