On Tue, 19 Nov 2013, Paul Burton wrote:
> Does anyone still care about the R2300? I ask because I'm working on
> the FP context switching code & noticed that I'm pretty sure the
> fpu_save_single & fpu_restore_single macros used only from
> r2300_switch.S are broken. They store each 32 bit value at the start
> of the location of the 64 bit FP registers context in memory, which I
> 1) Won't work for odd indexed FP registers with the FPU emulator,
> ptrace or other code which assumes that 32 bit FP data is held in
> the even-indexed 64 bit FP register context.
> 2) On big endian systems the 32 bit values will get saved to the most
> significant bits of the 64 bit context which I imagine will cause
> yet more problems.
> It seems like the only changes to r2300_switch.S for a *long* time have
> been to keep it in sync with r4k_switch.S & the CPU is old enough that
> all I get when I google for it is information about some hay baler.
> In short: does anyone care if I just submit a patch removing the R2300
> code instead of blindly attempting to fix it up?
Well, it worked the last time I tried (a couple of weeks ago) with actual
hardware (an R3400 integrated CPU/FPU), though maybe I missed something.
There hasn't been a lot of R2000/R3000-class hardware development recently
so no surprise our code didn't need any changes to match hardware updates.
At this point I see no reason to retire this code, there's nothing wrong
with it. If there's an actual bug, then it should be fixed. A test case
should be easy to make, and then we can start from there.
If you are concerned about register layout in ptrace packets, then please
see mips_read_fp_register_single and mips_read_fp_register_double in GDB
sources and the comment above them; notice the register buffer offset of 4
applied in the big-endian case -- what r2300_switch.S does is exactly what
the userland expects (of course it might be that r4k_switch.S is wrong in
some cases; actually I remember a discussion with Ralf where we came to
this very conclusion and rather than converting r4k_switch.S to use
LWC1/SWC1 -- that would degrade performance a bit for FP context switches
-- considered a helper to convert between the internal and the ptrace