MIPS: Indicate FP mode in sigcontext sc_used_math

The sc_used_math field of struct sigcontext & its variants has
traditionally been used as a boolean value indicating only whether or
not floating point context is saved within the sigcontext. With various
supported FP modes & the ability to switch between them this information
will no longer be enough to decode the meaning of the data stored in the
sc_fpregs fields of struct sigcontext.

To make that possible 3 bits are defined within sc_used_math:

- Bit 0 (USED_FP) represents whether FP was used, essentially
providing the boolean flag which sc_used_math as a whole provided
previously.

- Bit 1 (USED_FR1) provides the value of the Status.FR bit at the time
the FP context was saved.

- Bit 2 (USED_HYBRID_FPRS) indicates whether the FP context was saved
under the hybrid FPR scheme. Essentially, when set the odd singles
are located in bits 63:32 of the preceding even indexed sc_fpregs
element.

Any userland that tests whether the sc_used_math field is zero or
non-zero will continue to function as expected. Having said that, I
could not find any userland which uses the sc_used_math field at all.

[[e-mail]: Fixed rejects.]

Signed-off-by: Paul Burton <[e-mail]>
Cc: [e-mail]
Cc: Guenter Roeck <[e-mail]>
Cc: Matthew Fortune <[e-mail]>
Cc: Michael Ellerman <[e-mail]>
Cc: [e-mail]
Cc: Richard Weinberger <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Maciej W. Rozycki <[e-mail]>
Patchwork: https://patchwork.linux-mips.org/patch/10794/
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/include/uapi/asm/sigcontext.h

···
 #include <linux/types.h>
 #include <asm/sgidefs.h>
 
+/* scalar FP context was used */
+#define USED_FP			(1 << 0)
+
+/* the value of Status.FR when context was saved */
+#define USED_FR1		(1 << 1)
+
+/* FR=1, but with odd singles in bits 63:32 of preceding even double */
+#define USED_HYBRID_FPRS	(1 << 2)
+
 #if _MIPS_SIM == _MIPS_SIM_ABI32
 
 /*

arch/mips/kernel/signal.c

···
 	unsigned int used;
 	int err;
 
-	used = !!used_math();
+	used = used_math() ? USED_FP : 0;
+	if (used) {
+		if (!test_thread_flag(TIF_32BIT_FPREGS))
+			used |= USED_FR1;
+		if (test_thread_flag(TIF_HYBRID_FPREGS))
+			used |= USED_HYBRID_FPRS;
+	}
+
 	err = __put_user(used, used_math);
-	if (err || !used)
+	if (err || !(used & USED_FP))
 		return err;
 
 	/*
···
 	int err, sig, tmp __maybe_unused;
 
 	err = __get_user(used, used_math);
-	conditional_used_math(used);
+	conditional_used_math(used & USED_FP);
 
 	/*
 	 * The signal handler may have used FPU; give it up if the program
 	 * doesn't want it following sigreturn.
 	 */
-	if (err || !used) {
+	if (err || !(used & USED_FP)) {
 		lose_fpu(0);
 		return err;
 	}