[PATCH 1/3] arm64: Disable GiC priorities on Mediatek devices w/ firmware issues

[Douglas Anderson <dianders@chromium.org>]

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.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

 arch/arm64/kernel/cpufeature.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 2806a2850e78..e35efab8efa9 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2094,9 +2094,30 @@ static int __init early_enable_pseudo_nmi(char *p)
 }
 early_param("irqchip.gicv3_pseudo_nmi", early_enable_pseudo_nmi);
 
+static bool are_gic_priorities_broken(void)
+{
+	bool is_broken = false;
+	struct device_node *np;
+
+	/*
+	 * Detect broken Mediatek firmware that doesn't properly save and
+	 * restore GIC priorities.
+	 */
+	np = of_find_compatible_node(NULL, NULL, "arm,gic-v3");
+	if (np) {
+		is_broken = of_property_read_bool(np, "mediatek,broken-save-restore-fw");
+		of_node_put(np);
+	}
+
+	return is_broken;
+}
+
 static bool can_use_gic_priorities(const struct arm64_cpu_capabilities *entry,
 				   int scope)
 {
+	if (are_gic_priorities_broken())
+		return false;
+
 	/*
 	 * ARM64_HAS_GIC_CPUIF_SYSREGS has a lower index, and is a boot CPU
 	 * feature, so will be detected earlier.
-- 
2.42.0.609.gbb76f46606-goog