Re: PATCH: Always use ll/sc for mips

[Ralf Baechle <ralf at oss dot sgi dot com>]

On Thu, Jul 04, 2002 at 10:47:41AM +0200, Kevin D. Kissell wrote:

> The R5900 kernel for the Playstation 2 does not use system
> calls.  It uses a memory-mapped pseudo-device hack that
> the guys at Sony came up with, which is much faster.  We
> at MIPS came up with an even faster hack which uses 
> the destruction of a "k" register value, but which requires 
> the branch-likely instruction and thus only workson 
> MIPS II CPUs and above (R39xxx, R4xxx, R5xxx,
> but not the classic R3K).  See my message
> "Re: patches for test-and-set without ll/sc" of January 22.
> 
> I consider it to be very important for MIPS/Linux
> that the embedded/workstation kernel and libraries
> merge with the Playstation 2 "consumer" Linux, and
> I don't think that will happen if we try to push the
> PS2 people to use something far less efficient than
> what they already have. "Entia non sunt multiplicanda 
> praeter necessitatem", as a wise old guy once said,
> but could we not consider a MIPS/Linux universe
> where R3000 binaries use system calls, non-LL/SC
> MIPSII+ binaries use k-register destruction, real,
> manly, MIPS binaries use LL/SC instructions, and
> where the MIPS/Linux kernel (a) supports an appropriate
> system call, (b) makes a contract with userland to 
> destroy k-regs predictably, and (c) contains the
> emulation logic for LL/SC?  That should give us
> full cross-platform binary compatibility, with optimal
> performance on each platform when an appropriately
> configured set of libraries and tools is installed.

No, Sony's ABI isn't MP proof and will break silently on MP systems.  As
such I can't consider it anything else but a hack.  sysmips(MIPS_ATOMIC_SET,
...) and ll/sc however are MP proof.

  Ralf