fix(imx8mq): fix compilation with gcc >= 12.x

Starting with GCC >= 12.x the -Wall option includes -Werror=array-bounds
checks. Per default GCC treats all memory accesses below 4096 as NULL,
so access to ROMAPI causes the following warning:

------------
In file included from plat/imx/imx8m/imx8mq/imx8mq_bl31_setup.c:20:
In function 'mmio_read_8',
    inlined from 'imx8mq_soc_info_init' at plat/imx/imx8m/imx8mq/imx8mq_bl31_setup.c:70:16,
    inlined from 'bl31_platform_setup' at plat/imx/imx8m/imx8mq/imx8mq_bl31_setup.c:206:2:
include/lib/mmio.h:19:16: error: array subscript 0 is outside array bounds of 'volatile uint8_t[0]' {aka 'volatile unsigned char[]'} [-Werror=array-bounds]
   19 |         return *(volatile uint8_t*)addr;
      |                ^~~~~~~~~~~~~~~~~~~~~~~~
In function 'mmio_read_8',
    inlined from 'imx8mq_soc_info_init' at plat/imx/imx8m/imx8mq/imx8mq_bl31_setup.c:74:16,
    inlined from 'bl31_platform_setup' at plat/imx/imx8m/imx8mq/imx8mq_bl31_setup.c:206:2:
include/lib/mmio.h:19:16: error: array subscript 0 is outside array bounds of 'volatile uint8_t[0]' {aka 'volatile unsigned char[]'} [-Werror=array-bounds]
   19 |         return *(volatile uint8_t*)addr;
      |                ^~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
------------

This comes arguably from us somewhat abusing pointers to access MMIO
memory regions, which is not really covered by the C language.

Replace the pointer-dereferencing mmio_read_8() with an implementation
that uses inline assembly, to directly generate an 8-bit load
instruction. This avoids the compiler thinking that this access is using
a pointer it needs to jealously look after.

Change-Id: Iab39f6f615d51d3e8a1c54a1262d1e6ec208811d
Reported-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

1 files changed, 16 insertions, 2 deletions

diff --git a/plat/imx/imx8m/imx8mq/imx8mq_bl31_setup.c b/plat/imx/imx8m/imx8mq/imx8mq_bl31_setup.c
index 661f8e291..7065a6586 100644
--- a/plat/imx/imx8m/imx8mq/imx8mq_bl31_setup.c
+++ b/plat/imx/imx8m/imx8mq/imx8mq_bl31_setup.c
@@ -30,6 +30,20 @@
 
 #define TRUSTY_PARAMS_LEN_BYTES      (4096*2)
 
+/*
+ * Avoid the pointer dereference of the canonical mmio_read_8() implementation.
+ * This prevents the compiler from mis-interpreting the MMIO access as an
+ * illegal memory access to a very low address (the IMX ROM is mapped at 0).
+ */
+static uint8_t mmio_read_8_ldrb(uintptr_t address)
+{
+	uint8_t reg;
+
+	__asm__ volatile ("ldrb %w0, [%1]" : "=r" (reg) : "r" (address));
+
+	return reg;
+}
+
 static const mmap_region_t imx_mmap[] = {
 	MAP_REGION_FLAT(GPV_BASE, GPV_SIZE, MT_DEVICE | MT_RW), /* GPV map */
 	MAP_REGION_FLAT(IMX_ROM_BASE, IMX_ROM_SIZE, MT_MEMORY | MT_RO), /* ROM map */
@@ -70,11 +84,11 @@ static void imx8mq_soc_info_init(void)
 	uint32_t ocotp_val;
 
 	imx_soc_revision = mmio_read_32(IMX_ANAMIX_BASE + ANAMIX_DIGPROG);
-	rom_version = mmio_read_8(IMX_ROM_BASE + ROM_SOC_INFO_A0);
+	rom_version = mmio_read_8_ldrb(IMX_ROM_BASE + ROM_SOC_INFO_A0);
 	if (rom_version == 0x10)
 		return;
 
-	rom_version = mmio_read_8(IMX_ROM_BASE + ROM_SOC_INFO_B0);
+	rom_version = mmio_read_8_ldrb(IMX_ROM_BASE + ROM_SOC_INFO_B0);
 	if (rom_version == 0x20) {
 		imx_soc_revision &= ~0xff;
 		imx_soc_revision |= rom_version;