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SubjectRe: [PATCH] irqchip/gic-v3: Ensure GICR_CTLR.EnableLPI=0 is observed before enabling
Hi Shanker,

On Wed, 14 Mar 2018 00:50:01 +0000,
Shanker Donthineni 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>
> ---
> drivers/irqchip/irq-gic-v3-its.c | 30 +++++++++++++++++++++++-------
> include/linux/irqchip/arm-gic-v3.h | 1 +
> 2 files changed, 24 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> index 1d3056f..85cd158 100644
> --- a/drivers/irqchip/irq-gic-v3-its.c
> +++ b/drivers/irqchip/irq-gic-v3-its.c
> @@ -1875,15 +1875,31 @@ 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);
> + /* Make sure LPIs are disabled before programming PEND/PROP registers */
> + if (val & GICR_CTLR_ENABLE_LPIS) {
> + u32 count = 1000000; /* 1s! */
> +
> + /* 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.RWP==0 or timeout */
> + while (readl_relaxed(rbase + GICR_CTLR) & GICR_CTLR_RWP) {
> + if (!count) {
> + pr_err("CPU%d: Failed to disable LPIs\n",
> + smp_processor_id());
> + return;
> + }
> + cpu_relax();
> + udelay(1);
> + count--;
> + };
> + }

I can see a couple of issues with this patch:

- Entering the kernel with GICR_CTLR.EnableLPIs set is a recipe for
memory corruption and is likely to lead to Bad Things(tm). A loud
warning would be in order, I believe.

- If you're on a system that doesn't allow GICR_CTLR.Enable_LPIs to be
cleared, we end-up going down the polling path for nothing. It'd be
worth checking that the bit can be cleared the first place (and
shout again if it cannot).

- From a cosmetic PoV, please move this to a redist_disable_lpis()
function.

Thanks,

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

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