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[PATCH v3 7/7] arm64: docs: document AT_HWCAP2 and unused AT_HWCAP bits
- From: Andrew Murray <andrew dot murray at arm dot com>
- To: Catalin Marinas <catalin dot marinas at arm dot com>, Will Deacon <will dot deacon at arm dot com>
- Cc: Szabolcs Nagy <Szabolcs dot Nagy at arm dot com>, dave dot martin at arm dot com, linux-arm-kernel at lists dot infradead dot org, Mark Rutland <mark dot rutland at arm dot com>, Phil Blundell <pb at pbcl dot net>, libc-alpha at sourceware dot org, linux-api at vger dot kernel dot org
- Date: Mon, 1 Apr 2019 11:45:15 +0100
- Subject: [PATCH v3 7/7] arm64: docs: document AT_HWCAP2 and unused AT_HWCAP bits
- References: <email@example.com>
Now that we have expanded into AT_HWCAP2 let's add some documentation
to describe its presence. We also document the unused top half of
AT_HWCAP which we always return as 0 for userspace interoperation.
Signed-off-by: Andrew Murray <firstname.lastname@example.org>
Documentation/arm64/elf_hwcaps.txt | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)
diff --git a/Documentation/arm64/elf_hwcaps.txt b/Documentation/arm64/elf_hwcaps.txt
index c605757dd4db..6b3d4d334db7 100644
@@ -13,9 +13,9 @@ architected discovery mechanism available to userspace code at EL0. The
kernel exposes the presence of these features to userspace through a set
of flags called hwcaps, exposed in the auxilliary vector.
-Userspace software can test for features by acquiring the AT_HWCAP entry
-of the auxilliary vector, and testing whether the relevant flags are
+Userspace software can test for features by acquiring the AT_HWCAP or
+AT_HWCAP2 entry of the auxiliary vector, and testing whether the relevant
+flags are set, e.g.
@@ -198,3 +198,11 @@ HWCAP_PACG
Functionality implied by ID_AA64ISAR1_EL1.GPA == 0b0001 or
ID_AA64ISAR1_EL1.GPI == 0b0001, as described by
+4. Unused AT_HWCAP bits
+Each AT_HWCAP and AT_HWCAP2 entry provides for up to 32 hwcaps contained
+in bits [31:0]. For interoperation with userspace we guarantee that bit
+62 of AT_HWCAP will always be returned as 0.