acpigen: Support writing a length

It is convenient to write a length value for preceding a block of data.
Of course the length is not known or is hard to calculate a priori. So add
a way to mark the start on a stack, so the length can be updated when
known.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

lib/acpi/acpigen.c

@@ -10,6 +10,7 @@
 
 #include <common.h>
 #include <dm.h>
+#include <log.h>
 #include <acpi/acpigen.h>
 #include <dm/acpi.h>
 
@@ -38,6 +39,38 @@ void acpigen_emit_dword(struct acpi_ctx *ctx, uint data)
 	acpigen_emit_byte(ctx, (data >> 24) & 0xff);
 }
 
+/*
+ * Maximum length for an ACPI object generated by this code,
+ *
+ * If you need to change this, change acpigen_write_len_f(ctx) and
+ * acpigen_pop_len(ctx)
+ */
+#define ACPIGEN_MAXLEN 0xfffff
+
+void acpigen_write_len_f(struct acpi_ctx *ctx)
+{
+	assert(ctx->ltop < (ACPIGEN_LENSTACK_SIZE - 1));
+	ctx->len_stack[ctx->ltop++] = ctx->current;
+	acpigen_emit_byte(ctx, 0);
+	acpigen_emit_byte(ctx, 0);
+	acpigen_emit_byte(ctx, 0);
+}
+
+void acpigen_pop_len(struct acpi_ctx *ctx)
+{
+	int len;
+	char *p;
+
+	assert(ctx->ltop > 0);
+	p = ctx->len_stack[--ctx->ltop];
+	len = ctx->current - (void *)p;
+	assert(len <= ACPIGEN_MAXLEN);
+	/* generate store length for 0xfffff max */
+	p[0] = ACPI_PKG_LEN_3_BYTES | (len & 0xf);
+	p[1] = len >> 4 & 0xff;
+	p[2] = len >> 12 & 0xff;
+}
+
 void acpigen_emit_stream(struct acpi_ctx *ctx, const char *data, int size)
 {
 	int i;