Fix the Cortex-A57 reset handler register usage

The CPU specific reset handlers no longer have the freedom
of using any general purpose register because it is being invoked
by the BL3-1 entry point in addition to BL1. The Cortex-A57 CPU
specific reset handler was overwriting x20 register which was being
used by the BL3-1 entry point to save the entry point information.
This patch fixes this bug by reworking the register allocation in the
Cortex-A57 reset handler to avoid using x20. The patch also
explicitly mentions the register clobber list for each of the
callee functions invoked by the reset handler

Change-Id: I28fcff8e742aeed883eaec8f6c4ee2bd3fce30df

docs/firmware-design.md

@@ -1066,6 +1066,7 @@ array and returns it. Note that only the part number and implementator fields
 in midr are used to find the matching `cpu_ops` entry. The `reset_func()` in
 the returned `cpu_ops` is then invoked which executes the required reset
 handling for that CPU and also any errata workarounds enabled by the platform.
+This function must preserve the values of general purpose registers x20 to x29.
 
 Refer to Section "Guidelines for Reset Handlers" for general guidelines
 regarding placement of code in a reset handler.

docs/porting-guide.md

@@ -491,8 +491,7 @@ are just an ARM Trusted Firmware convention.
 A platform may need to do additional initialization after reset. This function
 allows the platform to do the platform specific intializations. Platform
 specific errata workarounds could also be implemented here. The api should
-preserve the value in x10 register as it is used by the caller to store the
+preserve the values of callee saved registers x19 to x29.
-return address.
 
 The default implementation doesn't do anything. If a platform needs to override
 the default implementation, refer to the [Firmware Design Guide] for general

lib/cpus/aarch64/cortex_a53.S

@@ -61,6 +61,7 @@ func cortex_a53_reset_func
 	/* ---------------------------------------------
 	 * As a bare minimum enable the SMP bit if it is
 	 * not already set.
+	 * Clobbers : x0
 	 * ---------------------------------------------
 	 */
 	mrs	x0, CPUECTLR_EL1

lib/cpus/aarch64/cortex_a57.S

@@ -87,6 +87,7 @@ func cortex_a57_disable_ext_debug
 	 * This applies only to revision r0p0 of Cortex A57.
 	 * Inputs:
 	 * x0: variant[4:7] and revision[0:3] of current cpu.
+	 * Clobbers : x0 - x5
 	 * --------------------------------------------------
 	 */
 func errata_a57_806969_wa
@@ -119,6 +120,7 @@ skip_806969:
 	 * This applies only to revision r0p0 of Cortex A57.
 	 * Inputs:
 	 * x0: variant[4:7] and revision[0:3] of current cpu.
+	 * Clobbers : x0 - x5
 	 * ---------------------------------------------------
 	 */
 func errata_a57_813420_wa
@@ -147,6 +149,7 @@ skip_813420:
 
 	/* -------------------------------------------------
 	 * The CPU Ops reset function for Cortex-A57.
+	 * Clobbers: x0-x5, x15, x19, x30
 	 * -------------------------------------------------
 	 */
 func cortex_a57_reset_func
@@ -155,20 +158,20 @@ func cortex_a57_reset_func
 
 	/*
 	 * Extract the variant[20:23] and revision[0:3] from x0
-	 * and pack it in x20[0:7] as variant[4:7] and revision[0:3].
+	 * and pack it in x15[0:7] as variant[4:7] and revision[0:3].
-	 * First extract x0[16:23] to x20[0:7] and zero fill the rest.
+	 * First extract x0[16:23] to x15[0:7] and zero fill the rest.
-	 * Then extract x0[0:3] into x20[0:3] retaining other bits.
+	 * Then extract x0[0:3] into x15[0:3] retaining other bits.
 	 */
-	ubfx	x20, x0, #(MIDR_VAR_SHIFT - MIDR_REV_BITS), #(MIDR_REV_BITS + MIDR_VAR_BITS)
+	ubfx	x15, x0, #(MIDR_VAR_SHIFT - MIDR_REV_BITS), #(MIDR_REV_BITS + MIDR_VAR_BITS)
-	bfxil	x20, x0, #MIDR_REV_SHIFT, #MIDR_REV_BITS
+	bfxil	x15, x0, #MIDR_REV_SHIFT, #MIDR_REV_BITS
 
 #if ERRATA_A57_806969
-	mov	x0, x20
+	mov	x0, x15
 	bl	errata_a57_806969_wa
 #endif
 
 #if ERRATA_A57_813420
-	mov	x0, x20
+	mov	x0, x15
 	bl	errata_a57_813420_wa
 #endif
 

lib/cpus/aarch64/cpu_helpers.S

@@ -42,11 +42,13 @@
 	 * The reset handler common to all platforms.  After a matching
 	 * cpu_ops structure entry is found, the correponding reset_handler
 	 * in the cpu_ops is invoked.
+	 * Clobbers: x0 - x19, x30
 	 */
 	.globl	reset_handler
 func reset_handler
 	mov	x19, x30
 
+	/* The plat_reset_handler can clobber x0 - x18, x30 */
 	bl	plat_reset_handler
 
 	/* Get the matching cpu_ops pointer */
@@ -60,6 +62,8 @@ func reset_handler
 	ldr	x2, [x0, #CPU_RESET_FUNC]
 	mov	x30, x19
 	cbz	x2, 1f
+
+	/* The cpu_ops reset handler can clobber x0 - x19, x30 */
 	br	x2
 1:
 	ret