[loong64] Port the RISC-V optimised TCP/IP checksum implementation

As with most other assembly code in iPXE, LoongArch64 is sufficiently
close to RISC-V that a straightforward transcription of the assembly
language code generally works.

Copy the RISC-V implementation for TCP/IP checksumming, retain the
register names and ABI, and just adjust the syntax to match
LoongArch64 requirements.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/loong64/core/loong64_tcpip.S

@@ -0,0 +1,127 @@
+/*
+ * Copyright (C) 2026 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ *
+ * You can also choose to distribute this program under the terms of
+ * the Unmodified Binary Distribution Licence (as given in the file
+ * COPYING.UBDL), provided that you have satisfied its requirements.
+ */
+
+	FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
+	FILE_SECBOOT ( PERMITTED )
+
+/** @file
+ *
+ * TCP/IP checksum
+ *
+ */
+
+	.section ".note.GNU-stack", "", @progbits
+	.text
+
+/**
+ * Calculate continued TCP/IP checkum
+ *
+ * @v partial		Checksum of already-summed data, in network byte order
+ * @v data		Data buffer
+ * @v len		Length of data buffer
+ * @ret cksum		Updated checksum, in network byte order
+ *
+ * In practice, this routine will only ever be called with a data
+ * pointer aligned to a 16-bit boundary.  We optimise for this case,
+ * ensuring that the code would still give correct output if called
+ * with a misaligned pointer.
+ */
+	.section ".text.tcpip_continue_chksum", "ax", @progbits
+	.globl	tcpip_continue_chksum
+tcpip_continue_chksum:
+
+	/* Set up register usage:
+	 *
+	 * a0: checksum low 64 bits
+	 * a1: data pointer
+	 * a2: end of data pointer
+	 * a3: end of data pointer minus a constant offset of interest
+	 * a4: checksum high bits (guaranteed to never carry)
+	 * a5: temporary register
+	 */
+	nor	$a0, $a0, $zero
+	add.d	$a2, $a2, $a1
+	addi.d	$a3, $a2, -8
+	move	$a4, $zero
+
+	/* Skip aligned checksumming if data is too short */
+	bgtu	$a1, $a3, post_aligned
+
+	/* Checksum 16-bit words until we reach 64-bit alignment (or
+	 * one byte past 64-bit alignment).
+	 */
+	b	2f
+1:	ld.hu	$a5, $a1, 0
+	addi.d	$a1, $a1, 2
+	add.d	$a4, $a4, $a5
+2:	andi	$a5, $a1, ( ( 8 - 1 ) & ~1 )
+	bnez	$a5, 1b
+
+	/* Checksum aligned 64-bit words */
+	b	2f
+1:	ld.d	$a5, $a1, 0
+	addi.d	$a1, $a1, 8
+	add.d	$a0, $a0, $a5
+	sltu	$a5, $a0, $a5
+	add.d	$a4, $a4, $a5
+2:	bleu	$a1, $a3, 1b
+
+post_aligned:
+	/* Checksum remaining 16-bit words */
+	addi.d	$a3, $a2, -2
+	b	2f
+1:	ld.hu	$a5, $a1, 0
+	addi.d	$a1, $a1, 2
+	add.d	$a4, $a4, $a5
+2:	bleu	$a1, $a3, 1b
+
+	/* Checksum final byte if present */
+	beq	$a1, $a2, 1f
+	ld.bu	$a5, $a1, 0
+	add.d	$a4, $a4, $a5
+1:
+	/* Fold down to 64 bits */
+	add.d	$a0, $a0, $a4
+	sltu	$a4, $a0, $a4
+	add.d	$a0, $a0, $a4
+
+	/* Fold down to (high) 32 bits */
+	slli.d	$a4, $a0, 32
+	add.d	$a0, $a0, $a4
+	sltu	$a4, $a0, $a4
+	slli.d	$a4, $a4, 32
+	add.d	$a0, $a0, $a4
+
+	/* Fold down to (high) 16 bits */
+	srli.d	$a4, $a0, 32
+	slli.d	$a4, $a4, 48
+	add.d	$a0, $a0, $a4
+	sltu	$a4, $a0, $a4
+	slli.d	$a4, $a4, 48
+	add.d	$a0, $a0, $a4
+
+	/* Negate, move to low bits, and return */
+	nor	$a0, $a0, $zero
+	srli.d	$a0, $a0, 48
+	ret
+	.size	tcpip_continue_chksum, . - tcpip_continue_chksum

src/arch/loong64/include/bits/tcpip.h

@@ -0,0 +1,16 @@
+#ifndef _BITS_TCPIP_H
+#define _BITS_TCPIP_H
+
+/** @file
+ *
+ * Transport-network layer interface
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+FILE_SECBOOT ( PERMITTED );
+
+extern uint16_t tcpip_continue_chksum ( uint16_t partial, const void *data,
+					size_t len );
+
+#endif /* _BITS_TCPIP_H */