Re: [PATCH v2 1/2] arm64: Move Mediatek GIC quirk handling from irqchip to core

In commit 44bd78dd2b88 ("irqchip/gic-v3: Disable pseudo NMIs on
Mediatek devices w/ firmware issues") we added a method for detecting
Mediatek devices with broken firmware and disabled pseudo-NMI. While
that worked, it didn't address the problem at a deep enough level.

The fundamental issue with this broken firmware is that it's not
saving and restoring several important GICR registers. The current
list is believed to be:
* GICR_NUM_IPRIORITYR
* GICR_CTLR
* GICR_ISPENDR0
* GICR_ISACTIVER0
* GICR_NSACR

Pseudo-NMI didn't work because it was the only thing (currently) in
the kernel that relied on the broken registers, so forcing pseudo-NMI
off was an effective fix. However, it could be observed that calling
system_uses_irq_prio_masking() on these systems still returned
"true". That caused confusion and led to the need for
commit a07a59415217 ("arm64: smp: avoid NMI IPIs with broken MediaTek
FW"). It's worried that the incorrect value returned by
system_uses_irq_prio_masking() on these systems will continue to
confuse future developers.

Let's fix the issue a little more completely by disabling IRQ
priorities at a deeper level in the kernel. Once we do this we can
revert some of the other bits of code dealing with this quirk.

This includes a partial revert of commit 44bd78dd2b88
("irqchip/gic-v3: Disable pseudo NMIs on Mediatek devices w/ firmware
issues"). This isn't a full revert because it leaves some of the
changes to the "quirks" structure around in case future code needs it.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

Changes in v2:
- Just detect the quirk once at init time.
- Fixed typo in subject: s/GiC/GIC.
- Squash in ("Remove Mediatek pseudo-NMI firmware quirk handling").

  arch/arm64/kernel/cpufeature.c | 46 ++++++++++++++++++++++++++++------
  drivers/irqchip/irq-gic-v3.c   | 22 +---------------
  2 files changed, 39 insertions(+), 29 deletions(-)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index f6b2e2906fc9..928124ea2e96 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c

@@ -999,6 +999,37 @@ static void init_32bit_cpu_features(struct cpuinfo_32bit *info)
  	init_cpu_ftr_reg(SYS_MVFR2_EL1, info->reg_mvfr2);
  }
  
+#ifdef CONFIG_ARM64_PSEUDO_NMI
+static bool enable_pseudo_nmi;
+
+static int __init early_enable_pseudo_nmi(char *p)
+{
+	return kstrtobool(p, &enable_pseudo_nmi);
+}
+early_param("irqchip.gicv3_pseudo_nmi", early_enable_pseudo_nmi);
+
+static __init void detect_system_supports_pseudo_nmi(void)
+{
+	struct device_node *np;
+
+	if (!enable_pseudo_nmi)
+		return;
+
+	/*
+	 * Detect broken Mediatek firmware that doesn't properly save and