OMAP3: Warning: swp{b} use is deprecated for this architecture

[will.deacon@arm.com (Will Deacon)]

Hi Jean,

> >>> Here is the offending code excerpt:
> >>>
> >>> wait_sem:
> >>>         mov     r0,#1
> >>>         ldr     r1, sdrc_scratchpad_sem
> >>> wait_loop:
> >>>         ldr     r2, [r1]        @ load the lock value
> >>>         cmp     r2, r0          @ is the lock free ?
> >>>         beq     wait_loop       @ not free...
> >>>         swp     r2, r0, [r1]    @ semaphore free so lock it and proceed
> >>>         cmp     r2, r0          @ did we succeed ?
> >>>         beq     wait_sem        @ no - try again
> >>
> >> (untested, as my LDP is useless because of OMAP regressions.)
> >>
> >> wait_sem:
> >>        mov     r0, #1
> >>        ldr     r1, sdrc_scratchpad_sem
> >> wait_loop:
> >>        ldrex   r2, [r1]        @ load lock value
> >>        teq     r2, r0          @ is lock free ( != 1)
> >>        beq     wait_loop       @ no, try again
> >>        strex   r2, r0, [r1]    @ try to lock
> >>        teq     r2, #0          @ did store succeed?
> >>        bne     wait_loop       @ no, try again
> >
> > I'm not familiar with the OMAP code but I recall they needed swp for
> > some synchronisation with external processor (DSP). Is this still the
> > case?
> This code is meant to be used by the new DVFS code which is not merged in yet.

I don't know what you do with this lock held or what the contention
is like, but you probably want a memory barrier immediately after you've
acquired it and another one before you release it.

Will