MIPS: Add IEEE Std 754 conformance mode selection

Add an `ieee754=' kernel parameter to control IEEE Std 754 conformance
mode.

Use separate flags copied from the respective CPU feature flags, and
adjusted according to the conformance mode selected, to make binaries
requesting individual NaN encoding modes accepted or rejected as needed.
Update the initial setting for FCSR and, in the full FPU emulation mode,
its read-only mask accordingly.  Accept the mode selection requested for
legacy processors as well.

As with the EF_MIPS_NAN2008 ELF file header flag adjust both ABS2008 and
NAN2008 bits at the same time, to match the choice made for hardware
currently implemented.

Signed-off-by: Maciej W. Rozycki <macro@imgtec.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Fortune <Matthew.Fortune@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/11481/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 35 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 742f69d18fc8..4bcd23355e86 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1436,6 +1436,41 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			In such case C2/C3 won't be used again.
 			idle=nomwait: Disable mwait for CPU C-states
 
+	ieee754=	[MIPS] Select IEEE Std 754 conformance mode
+			Format: { strict | legacy | 2008 | relaxed }
+			Default: strict
+
+			Choose which programs will be accepted for execution
+			based on the IEEE 754 NaN encoding(s) supported by
+			the FPU and the NaN encoding requested with the value
+			of an ELF file header flag individually set by each
+			binary.  Hardware implementations are permitted to
+			support either or both of the legacy and the 2008 NaN
+			encoding mode.
+
+			Available settings are as follows:
+			strict	accept binaries that request a NaN encoding
+				supported by the FPU
+			legacy	only accept legacy-NaN binaries, if supported
+				by the FPU
+			2008	only accept 2008-NaN binaries, if supported
+				by the FPU
+			relaxed	accept any binaries regardless of whether
+				supported by the FPU
+
+			The FPU emulator is always able to support both NaN
+			encodings, so if no FPU hardware is present or it has
+			been disabled with 'nofpu', then the settings of
+			'legacy' and '2008' strap the emulator accordingly,
+			'relaxed' straps the emulator for both legacy-NaN and
+			2008-NaN, whereas 'strict' enables legacy-NaN only on
+			legacy processors and both NaN encodings on MIPS32 or
+			MIPS64 CPUs.
+
+			The setting for ABS.fmt/NEG.fmt instruction execution
+			mode generally follows that for the NaN encoding,
+			except where unsupported by hardware.
+
 	ignore_loglevel	[KNL]
 			Ignore loglevel setting - this will print /all/
 			kernel messages to the console. Useful for debugging.