[RFC PATCH 2/2] ARM64: arch_timer: Work around QorIQ Erratum A-009971

[Scott Wood]

On Tue, 2016-04-12 at 09:22 +0100, Marc Zyngier wrote:
> On 12/04/16 06:54, Scott Wood wrote:
> > On Mon, 2016-04-11 at 10:55 +0100, Marc Zyngier wrote:
> > > On 11/04/16 03:22, Scott Wood wrote:
> > > > @@ -52,6 +53,20 @@ extern bool arm_arch_timer_reread;
> > > >  	_val; \
> > > >  })
> > > >  
> > > > +#define ARCH_TIMER_TVAL_REWRITE(pv, val) do { \
> > > > +	u64 _cnt_old, _cnt_new; \
> > > > +	int _timeout = 200; \
> > > > +	do { \
> > > > +		asm volatile("mrs %0, cntvct_el0;" \
> > > > +			     "msr cnt" pv "_tval_el0, %2;" \
> > > > +			     "mrs %1, cntvct_el0" \
> > > > +			     : "=&r" (_cnt_old), "=r" (_cnt_new) \
> > > > +			     : "r" (val)); \
> > > > +		_timeout--; \
> > > > +	} while (_cnt_old != _cnt_new && _timeout); \
> > > > +	WARN_ON_ONCE(!_timeout); \
> > > > +} while (0)
> > > > +
> > > 
> > > Given how many times you've written that loop, I'm sure you can have a
> > > preprocessor macro that will do the right thing.
> > 
> > I did use a preprocessor macro.  Are you asking me to consolidate the read
> > and
> > write macros?  That seems like it would create a mess that's worse than an
> > extra instance of a simple loop.
> 
> From patch 1:
> 
> +static __always_inline
> +u32 arch_timer_reg_tval_reread(int access, enum arch_timer_reg reg)
> +{
> +	u32 val, val_new;
> +	int timeout = 200;
> +
> +	do {
> +		if (access == ARCH_TIMER_PHYS_ACCESS) {
> +			asm volatile("mrc p15, 0, %0, c14, c2, 0;"
> +				     "mrc p15, 0, %1, c14, c2, 0"
> +				     : "=r" (val), "=r" (val_new));
> +		} else if (access == ARCH_TIMER_VIRT_ACCESS) {
> +			asm volatile("mrc p15, 0, %0, c14, c3, 0;"
> +				     "mrc p15, 0, %1, c14, c3, 0"
> +				     : "=r" (val), "=r" (val_new));
> +		}
> +		timeout--;
> +	} while (val != val_new && timeout);
> +
> +	WARN_ON_ONCE(!timeout);
> +	return val;
> +}
> 
> [...]
> 
> +static __always_inline u64 arch_counter_get_cnt(int opcode, bool reread)
>  {
> -	u64 cval;
> +	u64 val, val_new;
> +	int timeout = 200;
> 
>  	isb();
> -	asm volatile("mrrc p15, 0, %Q0, %R0, c14" : "=r" (cval));
> -	return cval;
> +
> +	if (reread) {
> +		do {
> +			asm volatile("mrrc p15, %2, %Q0, %R0, c14;"
> +				     "mrrc p15, %2, %Q1, %R1, c14"
> +				     : "=r" (val), "=r" (val_new)
> +				     : "i" (opcode));
> +			timeout--;
> +		} while (val != val_new && timeout);
> +
> +		BUG_ON(!timeout);
> +	} else {
> +		asm volatile("mrrc p15, %1, %Q0, %R0, c14" : "=r" (val)
> +			     : "i" (opcode));
> +	}
> +
> +	return val;
>  }
> 
> [...]
> 
> +/* QorIQ errata workarounds */
> +#define ARCH_TIMER_REREAD(reg) ({ \
> +	u64 _val_old, _val_new; \
> +	int _timeout = 200; \
> +	do { \
> +		asm volatile("mrs %0, " reg ";" \
> +			     "mrs %1, " reg \
> +			     : "=r" (_val_old), "=r" (_val_new)); \
> +		_timeout--; \
> +	} while (_val_old != _val_new && _timeout); \
> +	WARN_ON_ONCE(!_timeout); \
> +	_val_old; \
> +})
> 
> Do you notice a pattern? You are expressing the same loop in various
> ways (sometimes in a function, sometimes in a macro). I'm looking for a
> loop template that encapsulates the read access. You can have a similar
> macro for writes if you have more than a single instance.

One that covers arm and arm64?  Where would it go?

If you mean one per arch, that's already the case on 64-bit (and you
complained in response to the write macro, hence my inferring that you wanted
read and write combined).  Two instances on 32-bit (of a fairly simple loop)
didn't seem enough to warrant using ugly macros, but I can if you want (with
the entire asm statement passed as a macro parameter).

-Scott