lib/crypto: mips/chacha: Fix clang build and remove unneeded byteswap

The MIPS32r2 ChaCha code has never been buildable with the clang
assembler. First, clang doesn't support the 'rotl' pseudo-instruction:

error: unknown instruction, did you mean: rol, rotr?

Second, clang requires that both operands of the 'wsbh' instruction be
explicitly given:

error: too few operands for instruction

To fix this, align the code with the real instruction set by (1) using
the real instruction 'rotr' instead of the nonstandard pseudo-
instruction 'rotl', and (2) explicitly giving both operands to 'wsbh'.

To make removing the use of 'rotl' a bit easier, also remove the
unnecessary special-casing for big endian CPUs at
.Lchacha_mips_xor_bytes. The tail handling is actually
endian-independent since it processes one byte at a time. On big endian
CPUs the old code byte-swapped SAVED_X, then iterated through it in
reverse order. But the byteswap and reverse iteration canceled out.

Tested with chacha20poly1305-selftest in QEMU using "-M malta" with both
little endian and big endian mips32r2 kernels.

Fixes: 49aa7c00eddf ("crypto: mips/chacha - import 32r2 ChaCha code from Zinc")
Cc: [e-mail]
Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Eric Biggers <[e-mail]>

lib/crypto/mips/chacha-core.S

···
 #if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
 #define MSB 0
 #define LSB 3
-#define ROTx rotl
-#define ROTR(n) rotr n, 24
 #define	CPU_TO_LE32(n) \
-	wsbh	n; \
+	wsbh	n, n; \
 	rotr	n, 16;
 #else
 #define MSB 3
 #define LSB 0
-#define ROTx rotr
 #define CPU_TO_LE32(n)
-#define ROTR(n)
 #endif
 
 #define FOR_EACH_WORD(x) \
···
 	xor	X(W), X(B); \
 	xor	X(Y), X(C); \
 	xor	X(Z), X(D); \
-	rotl	X(V), S;    \
-	rotl	X(W), S;    \
-	rotl	X(Y), S;    \
-	rotl	X(Z), S;
+	rotr	X(V), 32 - S; \
+	rotr	X(W), 32 - S; \
+	rotr	X(Y), 32 - S; \
+	rotr	X(Z), 32 - S;
 
 .text
 .set	reorder
···
 	/* First byte */
 	lbu	T1, 0(IN)
 	addiu	$at, BYTES, 1
-	CPU_TO_LE32(SAVED_X)
-	ROTR(SAVED_X)
 	xor	T1, SAVED_X
 	sb	T1, 0(OUT)
 	beqz	$at, .Lchacha_mips_xor_done
 	/* Second byte */
 	lbu	T1, 1(IN)
 	addiu	$at, BYTES, 2
-	ROTx	SAVED_X, 8
+	rotr	SAVED_X, 8
 	xor	T1, SAVED_X
 	sb	T1, 1(OUT)
 	beqz	$at, .Lchacha_mips_xor_done
 	/* Third byte */
 	lbu	T1, 2(IN)
-	ROTx	SAVED_X, 8
+	rotr	SAVED_X, 8
 	xor	T1, SAVED_X
 	sb	T1, 2(OUT)
 	b	.Lchacha_mips_xor_done