plat/arm: Introduce ARM_LINUX_KERNEL_AS_BL33 build option

Normally, BL33 needs to contain a boot loader like U-Boot or UEFI that
eventually gives control to the OS. However, in some cases, this boot
sequence may be too slow. For example, when doing tests in a
cycle-accurate emulator, the user may only be interested in the
interaction between the Trusted Firmware and the OS, not in the boot
process itself.

The new option ARM_LINUX_KERNEL_AS_BL33 allows BL33 to contain the Linux
kernel image by changing the value of registers x0-x3 to the values
expected by the kernel. This option requires the device tree blob (DTB)
to be present in memory. Its address must be specified in the newly
introduced ARM_PRELOADED_DTB_BASE build option. For now, it only supports
AArch64 kernels.

This option is only available when RESET_TO_BL31=1. For this reason
the BL33 binary must be preloaded in memory and PRELOADED_BL33_BASE must
be used.

For example, if the kernel is loaded at 0x80080000 and the DTB is loaded
at address 0x82000000, the firmware could be built like this:

    CROSS_COMPILE=aarch64-linux-gnu-  \
    make PLAT=fvp DEBUG=1             \
    RESET_TO_BL31=1                   \
    ARM_LINUX_KERNEL_AS_BL33=1        \
    PRELOADED_BL33_BASE=0x80080000    \
    ARM_PRELOADED_DTB_BASE=0x82000000 \
    all fip

Change-Id: If9dc847c65ae2d0c27b51f0fd44fc06b28497db9
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

docs/user-guide.rst

@@ -678,6 +678,15 @@ Arm development platform specific build options
    Trusted Watchdog may be disabled at build time for testing or development
    purposes.
 
+-  ``ARM_LINUX_KERNEL_AS_BL33``: The Linux kernel expects registers x0-x3 to
+   have specific values at boot. This boolean option allows the Trusted Firmware
+   to have a Linux kernel image as BL33 by preparing the registers to these
+   values before jumping to BL33. This option defaults to 0 (disabled). For now,
+   it  only supports AArch64 kernels. ``RESET_TO_BL31`` must be 1 when using it.
+   If this option is set to 1, ``ARM_PRELOADED_DTB_BASE`` must be set to the
+   location of a device tree blob (DTB) already loaded in memory.  The Linux
+   Image address must be specified using the ``PRELOADED_BL33_BASE`` option.
+
 -  ``ARM_RECOM_STATE_ID_ENC``: The PSCI1.0 specification recommends an encoding
    for the construction of composite state-ID in the power-state parameter.
    The existing PSCI clients currently do not support this encoding of

plat/arm/common/arm_bl31_setup.c

@@ -80,7 +80,7 @@ void arm_bl31_early_platform_setup(bl31_params_t *from_bl2, uintptr_t soc_fw_con
 	assert(from_bl2 == NULL);
 	assert(plat_params_from_bl2 == NULL);
 
-#ifdef BL32_BASE
+# ifdef BL32_BASE
 	/* Populate entry point information for BL32 */
 	SET_PARAM_HEAD(&bl32_image_ep_info,
 				PARAM_EP,
@@ -89,7 +89,7 @@ void arm_bl31_early_platform_setup(bl31_params_t *from_bl2, uintptr_t soc_fw_con
 	SET_SECURITY_STATE(bl32_image_ep_info.h.attr, SECURE);
 	bl32_image_ep_info.pc = BL32_BASE;
 	bl32_image_ep_info.spsr = arm_get_spsr_for_bl32_entry();
-#endif /* BL32_BASE */
+# endif /* BL32_BASE */
 
 	/* Populate entry point information for BL33 */
 	SET_PARAM_HEAD(&bl33_image_ep_info,
@@ -105,6 +105,19 @@ void arm_bl31_early_platform_setup(bl31_params_t *from_bl2, uintptr_t soc_fw_con
 	bl33_image_ep_info.spsr = arm_get_spsr_for_bl33_entry();
 	SET_SECURITY_STATE(bl33_image_ep_info.h.attr, NON_SECURE);
 
+# if ARM_LINUX_KERNEL_AS_BL33
+	/*
+	 * According to the file ``Documentation/arm64/booting.txt`` of the
+	 * Linux kernel tree, Linux expects the physical address of the device
+	 * tree blob (DTB) in x0, while x1-x3 are reserved for future use and
+	 * must be 0.
+	 */
+	bl33_image_ep_info.args.arg0 = (u_register_t)ARM_PRELOADED_DTB_BASE;
+	bl33_image_ep_info.args.arg1 = 0U;
+	bl33_image_ep_info.args.arg2 = 0U;
+	bl33_image_ep_info.args.arg3 = 0U;
+# endif
+
 #else /* RESET_TO_BL31 */
 
 	/*

plat/arm/common/arm_common.mk

@@ -80,6 +80,27 @@ ARM_XLAT_TABLES_LIB_V1		:=	0
 $(eval $(call assert_boolean,ARM_XLAT_TABLES_LIB_V1))
 $(eval $(call add_define,ARM_XLAT_TABLES_LIB_V1))
 
+# Don't have the Linux kernel as a BL33 image by default
+ARM_LINUX_KERNEL_AS_BL33	:=	0
+$(eval $(call assert_boolean,ARM_LINUX_KERNEL_AS_BL33))
+$(eval $(call add_define,ARM_LINUX_KERNEL_AS_BL33))
+
+ifeq (${ARM_LINUX_KERNEL_AS_BL33},1)
+  ifneq (${ARCH},aarch64)
+    $(error "ARM_LINUX_KERNEL_AS_BL33 is only available in AArch64.")
+  endif
+  ifneq (${RESET_TO_BL31},1)
+    $(error "ARM_LINUX_KERNEL_AS_BL33 is only available if RESET_TO_BL31=1.")
+  endif
+  ifndef PRELOADED_BL33_BASE
+    $(error "PRELOADED_BL33_BASE must be set if ARM_LINUX_KERNEL_AS_BL33 is used.")
+  endif
+  ifndef ARM_PRELOADED_DTB_BASE
+    $(error "ARM_PRELOADED_DTB_BASE must be set if ARM_LINUX_KERNEL_AS_BL33 is used.")
+  endif
+  $(eval $(call add_define,ARM_PRELOADED_DTB_BASE))
+endif
+
 # Use an implementation of SHA-256 with a smaller memory footprint but reduced
 # speed.
 $(eval $(call add_define,MBEDTLS_SHA256_SMALLER))