docs: arm64: Document EL3 requirements for cpu debug architecture

This documents EL3 requirements for debug architecture. The register field
MDCR_EL3.TDA needs to be cleared for accesses into debug registers without
any trap being generated into EL3. CPU debug registers like DBGBCR<n>_EL1,
DBGBVR<n>_EL1, DBGWCR<n>_EL1, DBGWVR<n>_EL1 and MDSCR_EL1 are already being
accessed for HW breakpoint, watchpoint and debug monitor implementations on
the platform.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20241211065425.1106683-2-anshuman.khandual@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/Documentation/arch/arm64/booting.rst b/Documentation/arch/arm64/booting.rst
index 3278fb4bf219daab20a49e525f220f7d740c5836..1b3ac1394e5f404bf26d825f8e44e68fad3d67a1 100644 (file)
--- a/Documentation/arch/arm64/booting.rst
+++ b/Documentation/arch/arm64/booting.rst
@@ -449,6 +449,12 @@ Before jumping into the kernel, the following conditions must be met:
 
     - HFGWTR_EL2.nGCS_EL0 (bit 52) must be initialised to 0b1.
 
+ - For CPUs with debug architecture i.e FEAT_Debugv8pN (all versions):
+
+ - If EL3 is present:
+
+   - MDCR_EL3.TDA (bit 9) must be initialized to 0b0
+
 The requirements described above for CPU mode, caches, MMUs, architected
 timers, coherency and system registers apply to all CPUs.  All CPUs must
 enter the kernel in the same exception level.  Where the values documented