ASoC: SOF: special-case a part of the debug memory window

The debug memory window is split in 4KiB large slots. Slot zero is
used for slot descriptors, which leaves one or two more free slots
for actual useful work. One of them is usually used for mtrace
logging, which leaves no free slots on some systems and only one such
slot on others. This commit repurposes the free space in slot zero
after slot descriptors to be used as slot number 16, one higher than
the currently highest possible slot number.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Author: lyakh

include/sound/sof/ipc4/header.h

@@ -544,11 +544,19 @@ struct sof_ipc4_notify_resource_data {
 #define SOF_IPC4_DEBUG_DESCRIPTOR_SIZE		12 /* 3 x u32 */
 
 /*
- * The debug memory window is divided into 16 slots, and the
- * first slot is used as a recorder for the other 15 slots.
+ * The debug memory window is divided into up to 16 page-size slots, and slot
+ * zero is used as a recorder for the other slots.
+ * Note, that the last descriptor number 15 is used in a special way: so far on
+ * all ADSP platforms the debug memory window has 2 or 3 pages, i.e. it can only
+ * fit 1 or 2 slots in addition to the first slot with descriptors. But those
+ * descriptors occupy much less than 1kB of memory, so more than 3 kB of memory
+ * remain unused there. We use index 15 to point to those 3kB of memory in slot
+ * zero to be used for the GDB stub communication.
  */
-#define SOF_IPC4_MAX_DEBUG_SLOTS		15
-#define SOF_IPC4_DEBUG_SLOT_SIZE		0x1000
+#define SOF_IPC4_MAX_DEBUG_SLOTS	15				/* usable slots 1..15 */
+#define SOF_IPC4_MAX_DEBUG_DESCS	(SOF_IPC4_MAX_DEBUG_SLOTS + 1)	/* including descriptor
+									 * for partial slot 0 */
+#define SOF_IPC4_DEBUG_SLOT_SIZE	0x1000
 
 /* debug log slot types */
 #define SOF_IPC4_DEBUG_SLOT_UNUSED		0x00000000

sound/soc/sof/ipc4.c

@@ -586,11 +586,16 @@ size_t sof_ipc4_find_debug_slot_offset_by_type(struct snd_sof_dev *sdev,
 
 	/* The type is the second u32 in the slot descriptor */
 	slot_desc_type_offset = sdev->debug_box.offset + sizeof(u32);
-	for (i = 0; i < SOF_IPC4_MAX_DEBUG_SLOTS; i++) {
+	for (i = 0; i < SOF_IPC4_MAX_DEBUG_DESCS; i++) {
 		sof_mailbox_read(sdev, slot_desc_type_offset, &type, sizeof(type));
 
-		if (type == slot_type)
+		if (type == slot_type) {
+			if (i == SOF_IPC4_MAX_DEBUG_DESCS - 1)
+				/* Desc 15: see comment on SOF_IPC4_MAX_DEBUG_DESCS in header.h */
+				return sdev->debug_box.offset + 1024;
+
 			return sdev->debug_box.offset + (i + 1) * SOF_IPC4_DEBUG_SLOT_SIZE;
+		}
 
 		slot_desc_type_offset += SOF_IPC4_DEBUG_DESCRIPTOR_SIZE;
 	}