[dpdk-dev] [PATCH v13 2/5] eal: add the APIs to wait until equal

[Gavin Hu (Arm Technology China)]

> -----Original Message-----
> From: Jerin Jacob <jerinjacobk at gmail.com>
> Sent: Monday, November 11, 2019 1:12 PM
> To: Ananyev, Konstantin <konstantin.ananyev at intel.com>
> Cc: thomas at monjalon.net; David Marchand
> <david.marchand at redhat.com>; dev at dpdk.org; nd <nd at arm.com>; Gavin
> Hu (Arm Technology China) <Gavin.Hu at arm.com>; Mcnamara, John
> <john.mcnamara at intel.com>; Kovacevic, Marko
> <marko.kovacevic at intel.com>; jerinj at marvell.com; Jan Viktorin
> <viktorin at rehivetech.com>
> Subject: Re: [dpdk-dev] [PATCH v13 2/5] eal: add the APIs to wait until
> equal
> 
> On Sat, Nov 9, 2019 at 12:07 AM Ananyev, Konstantin
> <konstantin.ananyev at intel.com> wrote:
> >
> >
> >
> > > -----Original Message-----
> > > From: Thomas Monjalon <thomas at monjalon.net>
> > > Sent: Friday, November 8, 2019 5:00 PM
> > > To: Ananyev, Konstantin <konstantin.ananyev at intel.com>
> > > Cc: David Marchand <david.marchand at redhat.com>; dev at dpdk.org;
> nd at arm.com; Gavin Hu <gavin.hu at arm.com>; Mcnamara, John
> > > <john.mcnamara at intel.com>; Kovacevic, Marko
> <marko.kovacevic at intel.com>; Jerin Jacob <jerinj at marvell.com>; Jan
> Viktorin
> > > <viktorin at rehivetech.com>
> > > Subject: Re: [PATCH v13 2/5] eal: add the APIs to wait until equal
> > >
> > > 08/11/2019 17:38, Ananyev, Konstantin:
> > > > > From: Gavin Hu <gavin.hu at arm.com>
> > > > > +static __rte_always_inline void
> > > > > +rte_wait_until_equal_64(volatile uint64_t *addr, uint64_t expected,
> > > > > +         int memorder)
> > > > > +{
> > > > > + uint64_t value;
> > > > > +
> > > > > + assert(memorder == __ATOMIC_ACQUIRE || memorder ==
> __ATOMIC_RELAXED);
> > > > > +
> > > > > + /*
> > > > > +  * Atomic exclusive load from addr, it returns the 64-bit content of
> > > > > +  * *addr while making it 'monitored',when it is written by someone
> > > > > +  * else, the 'monitored' state is cleared and a event is generated
> > > > > +  * implicitly to exit WFE.
> > > > > +  */
> > > > > +#define __LOAD_EXC_64(src, dst, memorder) {              \
> > > > > + if (memorder == __ATOMIC_RELAXED) {              \
> > > > > +         asm volatile("ldxr %x[tmp], [%x[addr]]"  \
> > > > > +                 : [tmp] "=&r" (dst)              \
> > > > > +                 : [addr] "r"(src)                \
> > > > > +                 : "memory");                     \
> > > > > + } else {                                         \
> > > > > +         asm volatile("ldaxr %x[tmp], [%x[addr]]" \
> > > > > +                 : [tmp] "=&r" (dst)              \
> > > > > +                 : [addr] "r"(src)                \
> > > > > +                 : "memory");                     \
> > > > > + } }
> > > > > +
> > > > > + __LOAD_EXC_64(addr, value, memorder)
> > > > > + if (value != expected) {
> > > > > +         __SEVL()
> > > > > +         do {
> > > > > +                 __WFE()
> > > > > +                 __LOAD_EXC_64(addr, value, memorder)
> > > > > +         } while (value != expected);
> > > > > + }
> > > > > +}
> > > > > +#undef __LOAD_EXC_64
> > > > > +
> > > > > +#undef __SEVL
> > > > > +#undef __WFE
> > > >
> > > > Personally I don't see how these define/undef are better then inline
> functions...
> > >
> > > The benefit it to not pollute the API namespace
> > > with some functions which are used only locally.
> > > After using a macro, it disappears with #undef.
> > >
> > > > Again I suppose they might be re-used in future some other ARM
> specific low-level primitvies.
> > >
> > > Do this low-level routines _LOAD_EXC_ need to be exposed in EAL for re-
> use?
> >
> > About load_exc don't know for sure...
> > Though as I can see wfe/sevl are used here:
> > http://patchwork.dpdk.org/patch/61779/
> > Might be it is possible to reuse these functions/macros...
> > But again, I am not arm expert, would be interested to know what arm
> guys think.
> 
> 
> Considering WFE has a requirement on using load_exec on ARM so IMO, it
> makes sense to expose
> load_exec in conjunction with wfe/sevl to use it properly.
Agree, WFE should have more use cases can be developed, not limited to the WAIT_UNTIL_EQUAL_XX APIs. Marvel's patch is an example.
Sorry I don't have more use case so far, but Arm is evolving WFE, so I think more use cases are emerging. 
/Gavin
> 
> 
> >
> > >
> > > > My preference would be to keep them as inline function - much
> cleaner code.
> > > > But as I don't develop for/use, I wouldn't insist and let you and arm
> guys to decide.
> > >
> > >
> >