dmix: add S24_3LE support

Add to the dmix plugin support for the S24_3LE sample format which is
used by 24-bit USB devices.

The optimized assembler version uses only 23 bits for sample data so
that the lowest bit can be used for synchronization because there is no
24-bit cmpxchg instruction.

src/pcm/pcm_dmix_x86_64.h

@@ -237,3 +237,105 @@ static void MIX_AREAS2(unsigned int size,
 	);
 }
 
+/*
+ *  24-bit version
+ */
+static void MIX_AREAS3(unsigned int size,
+		       volatile unsigned char *dst, unsigned char *src,
+		       volatile signed int *sum, size_t dst_step,
+		       size_t src_step, size_t sum_step)
+{
+	unsigned long long old_rbx;
+
+	/*
+	 *  RSI - src
+	 *  RDI - dst
+	 *  RBX - sum
+	 *  ECX - old sample
+	 *  EAX - sample / temporary
+	 *  EDX - temporary
+	 */
+	__asm__ __volatile__ (
+		"\n"
+
+		"\tmovq %%rbx, %7\n"
+		/*
+		 *  initialization, load ESI, EDI, EBX registers
+		 */
+		"\tmovq %1, %%rdi\n"
+		"\tmovq %2, %%rsi\n"
+		"\tmovq %3, %%rbx\n"
+
+		/*
+		 * while (size-- > 0) {
+		 */
+		"\tcmpl $0, %0\n"
+		"jz 6f\n"
+
+		"\t.p2align 4,,15\n"
+
+		"1:"
+
+		/*
+		 *   sample = *src;
+		 *   sum_sample = *sum;
+		 *   if (test_and_set_bit(0, dst) == 0)
+		 *     sample -= sum_sample;
+		 *   *sum += sample;
+		 */
+		"\tmovsbl 2(%%rsi), %%eax\n"
+		"\tmovswl (%%rsi), %%ecx\n"
+		"\tmovl (%%rbx), %%edx\n"
+		"\tsall $16, %%eax\n"
+		"\t" LOCK_PREFIX "btsl $0, (%%rdi)\n"
+		"\tleal (%%ecx,%%eax,1), %%ecx\n"
+		"\tjc 2f\n"
+		"\tsubl %%edx, %%ecx\n"
+		"2:"
+		"\t" LOCK_PREFIX "addl %%ecx, (%%rbx)\n"
+
+		/*
+		 *   do {
+		 *     sample = old_sample = *sum;
+		 *     saturate(sample);
+		 *     *dst = sample | 1;
+		 *   } while (old_sample != *sum);
+		 */
+
+		"3:"
+		"\tmovl (%%rbx), %%ecx\n"
+
+		"\tmovl $0x7fffff, %%eax\n"
+		"\tmovl $-0x7fffff, %%edx\n"
+		"\tcmpl %%eax, %%ecx\n"
+		"\tcmovng %%ecx, %%eax\n"
+		"\tcmpl %%edx, %%ecx\n"
+		"\tcmovl %%edx, %%eax\n"
+
+		"\torl $1, %%eax\n"
+		"\tmovw %%ax, (%%rdi)\n"
+		"\tshrl $16, %%eax\n"
+		"\tmovb %%al, 2(%%rdi)\n"
+	
+		"\tcmpl %%ecx, (%%rbx)\n"
+		"\tjnz 3b\n"
+
+		/*
+		 * while (size-- > 0)
+		 */
+		"\tadd %4, %%rdi\n"
+		"\tadd %5, %%rsi\n"
+		"\tadd %6, %%rbx\n"
+		"\tdecl %0\n"
+		"\tjnz 1b\n"
+		
+		"6:"
+		"\tmovq %7, %%rbx\n"
+
+		: /* no output regs */
+		: "m" (size), "m" (dst), "m" (src),
+		  "m" (sum), "m" (dst_step), "m" (src_step),
+		  "m" (sum_step), "m" (old_rbx)
+		: "rsi", "rdi", "edx", "ecx", "eax"
+	);
+}