irqchip/gic-v3: Probe for SCR_EL3 being clear before resetting AP0Rn

We would like to reset the Group-0 Active Priority Registers
at boot time if they are available to us. They would be available
if SCR_EL3.FIQ was not set, but we cannot directly probe this bit,
and short of checking, we may end-up trapping to EL3, and the
firmware may not be please to get such an exception. Yes, this
is dumb.

Instead, let's use PMR to find out if its value gets affected by
SCR_EL3.FIQ being set. We use the fact that when SCR_EL3.FIQ is
set, the LSB of the priority is lost due to the shifting back and
forth of the actual priority. If we read back a 0, we know that
Group0 is unavailable. In case we read a non-zero value, we can
safely reset the AP0Rn register.
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>

arch/arm/include/asm/arch_gicv3.h

@@ -137,6 +137,7 @@ static inline u64 read_ ## a64(void)		\
 	return val; 				\
 }
 
+CPUIF_MAP(ICC_PMR, ICC_PMR_EL1)
 CPUIF_MAP(ICC_AP0R0, ICC_AP0R0_EL1)
 CPUIF_MAP(ICC_AP0R1, ICC_AP0R1_EL1)
 CPUIF_MAP(ICC_AP0R2, ICC_AP0R2_EL1)
@@ -206,11 +207,6 @@ static inline u32 gic_read_iar(void)
 	return irqstat;
 }
 
-static inline void gic_write_pmr(u32 val)
-{
-	write_sysreg(val, ICC_PMR);
-}
-
 static inline void gic_write_ctlr(u32 val)
 {
 	write_sysreg(val, ICC_CTLR);

arch/arm64/include/asm/arch_gicv3.h

@@ -76,11 +76,6 @@ static inline u64 gic_read_iar_cavium_thunderx(void)
 	return irqstat;
 }
 
-static inline void gic_write_pmr(u32 val)
-{
-	write_sysreg_s(val, SYS_ICC_PMR_EL1);
-}
-
 static inline void gic_write_ctlr(u32 val)
 {
 	write_sysreg_s(val, SYS_ICC_CTLR_EL1);

drivers/irqchip/irq-gic-v3.c

@@ -532,7 +532,8 @@ static void gic_cpu_sys_reg_init(void)
 	int i, cpu = smp_processor_id();
 	u64 mpidr = cpu_logical_map(cpu);
 	u64 need_rss = MPIDR_RS(mpidr);
-	u32 val;
+	bool group0;
+	u32 val, pribits;
 
 	/*
 	 * Need to check that the SRE bit has actually been set. If
@@ -544,8 +545,28 @@ static void gic_cpu_sys_reg_init(void)
 	if (!gic_enable_sre())
 		pr_err("GIC: unable to set SRE (disabled at EL2), panic ahead\n");
 
+	pribits = gic_read_ctlr();
+	pribits &= ICC_CTLR_EL1_PRI_BITS_MASK;
+	pribits >>= ICC_CTLR_EL1_PRI_BITS_SHIFT;
+	pribits++;
+
+	/*
+	 * Let's find out if Group0 is under control of EL3 or not by
+	 * setting the highest possible, non-zero priority in PMR.
+	 *
+	 * If SCR_EL3.FIQ is set, the priority gets shifted down in
+	 * order for the CPU interface to set bit 7, and keep the
+	 * actual priority in the non-secure range. In the process, it
+	 * looses the least significant bit and the actual priority
+	 * becomes 0x80. Reading it back returns 0, indicating that
+	 * we're don't have access to Group0.
+	 */
+	write_gicreg(BIT(8 - pribits), ICC_PMR_EL1);
+	val = read_gicreg(ICC_PMR_EL1);
+	group0 = val != 0;
+
 	/* Set priority mask register */
-	gic_write_pmr(DEFAULT_PMR_VALUE);
+	write_gicreg(DEFAULT_PMR_VALUE, ICC_PMR_EL1);
 
 	/*
 	 * Some firmwares hand over to the kernel with the BPR changed from
@@ -563,11 +584,24 @@ static void gic_cpu_sys_reg_init(void)
 		gic_write_ctlr(ICC_CTLR_EL1_EOImode_drop_dir);
 	}
 
-	val = gic_read_ctlr();
-	val &= ICC_CTLR_EL1_PRI_BITS_MASK;
-	val >>= ICC_CTLR_EL1_PRI_BITS_SHIFT;
+	/* Always whack Group0 before Group1 */
+	if (group0) {
+		switch(pribits) {
+		case 8:
+		case 7:
+			write_gicreg(0, ICC_AP0R3_EL1);
+			write_gicreg(0, ICC_AP0R2_EL1);
+		case 6:
+			write_gicreg(0, ICC_AP0R1_EL1);
+		case 5:
+		case 4:
+			write_gicreg(0, ICC_AP0R0_EL1);
+		}
+
+		isb();
+	}
 
-	switch(val + 1) {
+	switch(pribits) {
 	case 8:
 	case 7:
 		write_gicreg(0, ICC_AP1R3_EL1);