Re: [PATCH v2] irqchip/gic-v3: Ensure GICR_CTLR.EnableLPI=0 is observed before enabling

[Marc Zyngier <marc.zyngier@arm.com>]

On Sat, 17 Mar 2018 16:11:06 +0000,
Shanker Donthineni wrote:
> 
> Hi Marc,
> 
> On 03/17/2018 08:34 AM, Marc Zyngier wrote:
> > On Thu, 15 Mar 2018 09:31:27 -0500
> > Shanker Donthineni <shankerd@codeaurora.org> wrote:
> > 
> >> The definition of the GICR_CTLR.RWP control bit was expanded to indicate
> >> status of changing GICR_CTLR.EnableLPI from 1 to 0 is being in progress
> >> or completed. Software must observe GICR_CTLR.RWP==0 after clearing
> >> GICR_CTLR.EnableLPI from 1 to 0 and before writing GICR_PENDBASER and/or
> >> GICR_PROPBASER, otherwise behavior is UNPREDICTABLE.
> >>
> >> Signed-off-by: Shanker Donthineni <shankerd@codeaurora.org>
> >> ---
> >> Changes since v1:
> >>   -Moved LPI disable code to a seperate function as Marc suggested.
> >>   -Mark's suggestion to use readl_relaxed_poll_timeout() helper functions.
> >>   
> >>  drivers/irqchip/irq-gic-v3-its.c   | 66 ++++++++++++++++++++++++++++++--------
> >>  include/linux/irqchip/arm-gic-v3.h |  1 +
> >>  2 files changed, 53 insertions(+), 14 deletions(-)
> >>
> >> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> >> index 1d3056f..cba71a7 100644
> >> --- a/drivers/irqchip/irq-gic-v3-its.c
> >> +++ b/drivers/irqchip/irq-gic-v3-its.c
> >> @@ -33,6 +33,8 @@
> >>  #include <linux/of_platform.h>
> >>  #include <linux/percpu.h>
> >>  #include <linux/slab.h>
> >> +#include <linux/time64.h>
> >> +#include <linux/iopoll.h>
> >>  
> >>  #include <linux/irqchip.h>
> >>  #include <linux/irqchip/arm-gic-v3.h>
> >> @@ -1875,16 +1877,6 @@ static void its_cpu_init_lpis(void)
> >>  		gic_data_rdist()->pend_page = pend_page;
> >>  	}
> >>  
> >> -	/* Disable LPIs */
> >> -	val = readl_relaxed(rbase + GICR_CTLR);
> >> -	val &= ~GICR_CTLR_ENABLE_LPIS;
> >> -	writel_relaxed(val, rbase + GICR_CTLR);
> >> -
> >> -	/*
> >> -	 * Make sure any change to the table is observable by the GIC.
> >> -	 */
> >> -	dsb(sy);
> >> -
> >>  	/* set PROPBASE */
> >>  	val = (page_to_phys(gic_rdists->prop_page) |
> >>  	       GICR_PROPBASER_InnerShareable |
> >> @@ -3288,13 +3280,59 @@ static bool gic_rdists_supports_plpis(void)
> >>  	return !!(gic_read_typer(gic_data_rdist_rd_base() + GICR_TYPER) & GICR_TYPER_PLPIS);
> >>  }
> >>  
> >> +static int redist_disable_lpis(void)
> >> +{
> >> +	void __iomem *rbase = gic_data_rdist_rd_base();
> >> +	u64 val;
> >> +	int ret;
> >> +
> >> +	if (!gic_rdists_supports_plpis()) {
> >> +		pr_info("CPU%d: LPIs not supported\n", smp_processor_id());
> >> +		return -ENXIO;
> >> +	}
> >> +
> >> +	val = readl_relaxed(rbase + GICR_CTLR);
> >> +	if (!(val & GICR_CTLR_ENABLE_LPIS))
> >> +		return 0;
> >> +
> >> +	/* Disable LPIs */
> >> +	val &= ~GICR_CTLR_ENABLE_LPIS;
> >> +	writel_relaxed(val, rbase + GICR_CTLR);
> >> +
> >> +	/* Make sure any change to GICR_CTLR is observable by the GIC */
> >> +	dsb(sy);
> >> +
> >> +	/* Wait for GICR_CTLR.GICR_CTLR_ENABLE_LPIS==0 or timeout */
> >> +	ret = readl_relaxed_poll_timeout_atomic(rbase + GICR_CTLR, val,
> >> +					 !(val & GICR_CTLR_ENABLE_LPIS), 1,
> >> +					 USEC_PER_SEC);
> > 
> > Why do you need to poll on EnableLPIs? It is supposed to be immediately
> > written. It seems to me that the sequence should be:
> > 
> 
> Agree we don't need to poll EnableLPIs here. I'll remove.
> 
> > - Check EnableLPis
> > - If set to 1, scream about bootloader being braindead
> > - Write EnableLPIs to 0
> > - Read back EnableLpis:
> > - If still set 1, bail out, we're doomed. Scream about the HW being
> >   braindead
> > - Poll RWP
> > - If timeout, bail out, the HW has locked up. Scream again.
> > 
> > I think this should cover the whole thing. An alternative would be to
> > place the poll between the write and read-back, but that doesn't change
> > anything.
> > 
> 
> I'll prefer your second approach. Please comment on the below change before
> I post v3.
> 
> static int redist_disable_lpis(void)
> {
>         void __iomem *rbase = gic_data_rdist_rd_base();
>         u64 val;
>         int ret;
> 
>         if (!gic_rdists_supports_plpis()) {
>                 pr_info("CPU%d: LPIs not supported\n", smp_processor_id());
>                 return -ENXIO;
>         }
> 
>         if (!(readl(rbase + GICR_CTLR) & GICR_CTLR_ENABLE_LPIS))
>                 return 0;
> 
>         pr_warn("CPU%d: Trying to disable LPIs\n", smp_processor_id());

I'd be much more clear:

	pr_warn("CPU%d: Booted with LPIs enabled, memory probably corrupted\n",
		smp_processor_id());
	add_taint(TAINT_CRAP, LOCKDEP_STILL_OK);

>         val &= ~GICR_CTLR_ENABLE_LPIS;
>         writel_relaxed(val, rbase + GICR_CTLR);
> 
>         /* Make sure any change to GICR_CTLR is observable by the GIC */
>         dsb(sy);
> 
>         /**
>          * Software must observe RWP==0 after clearing GICR_CTLR.EnableLPIs
>          * from 1 to 0 before programming GICR_PEND{PROP}BASER registers.
>          * Bail out the driver probe() in case of timeout.
>          */
>         ret = readl_relaxed_poll_timeout_atomic(rbase + GICR_CTLR, val,
>                                                 !(val & GICR_CTLR_RWP), 1,
>                                                 USEC_PER_SEC);
>         if (ret) {
>                 pr_err("CPU%d: EnableLPIs not observed\n", smp_processor_id());

Let's call a spade a spade: the redistributor hasn't reacted after a
whole second, it has probably locked-up. Just say so, because your
message doesn't mean much to me.

>                 return ret;
>         }
>  
>         /**
>          * After it has been written to 1, it is IMPLEMENTATION DEFINED whether
>          * the bit GICR_CTLR.EnableLPI becomes RES1 or can be cleared to 0.
>          * Bail out the driver probe() on systems where it's RES1.
>          */
>         if (readl_relaxed(rbase + GICR_CTLR) & GICR_CTLR_ENABLE_LPIS) {
>                 pr_err("CPU%d: Failed to disable LPIs\n", smp_processor_id());
>                 return -EPERM;

EPERM? EBUSY was a better approximation...

Thanks,

	M.

-- 
Jazz is not dead, it just smell funny.