ASoC: SOF: ipc4-mtrace: resync host_read_ptr on debugfs open

core_data->dsp_write_ptr is updated on each aging timer IPC, but there
is 256ms window between DSP write and IPC. If reader opens in this window
and DSP goes idle before next IPC,there wont be further writes
and sof_wait_mtrace_avail() blocks until timeout.

Fix by reading dsp_write_ptr directly from SRAM in open() to get
current write position without waiting for the next IPC. host_read_ptr
is left unchanged to preserve inter-session data.

Signed-off-by: Mateusz Junkier <mateusz.junkier@intel.com>

Author: majunkier

sound/soc/sof/ipc4-mtrace.c

@@ -109,6 +109,20 @@ static int sof_ipc4_mtrace_dfs_open(struct inode *inode, struct file *file)
 		return -ENOMEM;
 	}
 
+	/*
+	* Re-sync dsp_write_ptr from SRAM on open to avoid blocking when
+	* DSP goes idle before next aging timer IPC arrives.
+	*/
+	if (core_data->slot_offset != SOF_IPC4_INVALID_SLOT_OFFSET) {
+		struct snd_sof_dev *sdev = core_data->sdev;
+		u32 write_ptr;
+
+		sof_mailbox_read(sdev, core_data->slot_offset + sizeof(u32),
+				 &write_ptr, sizeof(write_ptr));
+		write_ptr -= write_ptr % 4;
+		core_data->dsp_write_ptr = write_ptr;
+	}
+
 	ret = simple_open(inode, file);
 	if (ret) {
 		kfree(core_data->log_buffer);