linux-kernel - Re: [PATCH] ARM: supplementing IO accessors with 64 bit capability

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Date:	Mon, 27 Oct 2014 15:54:45 +0000
From:	Will Deacon <will.deacon@....com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Mathieu Poirier <mathieu.poirier@...aro.org>,
	"thomas.petazzoni@...e-electrons.com" 
	<thomas.petazzoni@...e-electrons.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"stefano.stabellini@...citrix.com" <stefano.stabellini@...citrix.com>,
	Liviu Dudau <Liviu.Dudau@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ezequiel.garcia@...e-electrons.com" 
	<ezequiel.garcia@...e-electrons.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: supplementing IO accessors with 64 bit capability

On Fri, Oct 24, 2014 at 05:16:34PM +0100, Catalin Marinas wrote:
> On Fri, Oct 24, 2014 at 04:05:13PM +0100, Mathieu Poirier wrote:
> > On 24 October 2014 03:28, Catalin Marinas <catalin.marinas@....com> wrote:
> > > On Wed, Oct 22, 2014 at 08:10:27PM +0100, Mathieu Poirier wrote:
> > >> On 22 October 2014 18:44, Catalin Marinas <catalin.marinas@....com> wrote:
> > >> > On Wed, Oct 22, 2014 at 05:06:23PM +0100, mathieu.poirier@...aro.org wrote:
> > >> >> +static inline void __raw_writeq(u64 val, volatile void __iomem *addr)
> > >> >> +{
> > >> >> +     asm volatile("strd %1, %0"
> > >> >> +                  : "+Qo" (*(volatile u64 __force *)addr)
> > >> >> +                  : "r" (val));
> > >> >> +}
> > >> >> +
> > >> >> +static inline u64 __raw_readq(const volatile void __iomem *addr)
> > >> >> +{
> > >> >> +     u64 val;
> > >> >> +     asm volatile("ldrd %1, %0"
> > >> >> +                  : "+Qo" (*(volatile u64 __force *)addr),
> > >> >> +                    "=r" (val));
> > >> >> +     return val;
> > >> >> +}
> > >> >> +#endif
> > >> >
> > >> > I'm curious why you need these. Do you have a device that needs a 64-bit
> > >> > single access or you are trying to read two consecutive registers?
> > >>
> > >> The fundamental data size of Coresight STM32 for ARMv7 is
> > >> implementation defined and can be 32 or 64bit.  As such stimulus ports
> > >> can support transaction sizes of up to 64 bit.
> > >
> > > The STM programmer's model spec recommends something else (though I find
> > > the "3.6 Data sizes" chapter a bit confusing):
> > >
> > >   To ensure that code is portable between processor micro-architectures
> > >   and system implementations, ARM recommends that only the native data
> > >   size of the machine is used, and smaller sizes. For the 32-bit ARMv7
> > >   architecture, only 8, 16, and 32-bit transfers are recommended. For an
> > >   ARMv8 processor that supports the AArch64 Execution state, it is
> > >   recommended that the fundamental data size of 64-bits is implemented.
> > >
> > > Which means that you should not use readq/writeq on a 32-bit system.
> > 
> > Not quite.  ARM documentation IHI0054B (ARM System Trace Macrocell:
> > Programmers' Model Architecture Specification) stipulate that "For
> > systems with an ARMv7 processor, ARM recommends configuration 1 or
> > configuration 2.", where configuration 2 has a fundamental size of 64
> > bit.
> 
> As I said, it's confusing. Anyway, you can go ahead and add the
> readq/writeq for ARMv6 and later, though it won't be guaranteed to have
> a 64-bit access, it depends on the bus.

I'm really not comfortable with this... we don't make any guarantees for
32-bit CPUs that a double-word access will be single-copy atomic for MMIO.
That means it could be subjected to things like reordering and merging,
which I think means that it depends on both the bus *and* the endpoint as to
whether or not this will work. Worse still, the endpoint could decide to
return a SLVERR, which would appear as an external abort.

Is it not possible to use 32-bit MMIO accesses for this driver?

Will
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