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

IPC4 mtrace aging timer sends NOTIFY_LOG_BUFFER_STATUS IPC every 256ms
but only when dsp_write_ptr has changed since last tick. If IPC
is sent while no reader is active, the wake_up() is lost and
core_data->dsp_write_ptr becomes stale.

Open() finds host_read_ptr == dsp_write_ptr (both
reflecting stale cached value) and blocks in sof_wait_mtrace_avail()
waiting for IPC that will never come since DSP has gone idle.
This causes empty trace output on platforms with low post-boot DSP
activity.

Fix this by re-reading dsp_write_ptr directly from SRAM in open() so
reader see current write position without relying on IPC cache.
host_read_ptr is intentionally left unchanged to preserve any
data written between sessions

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 the aging timer IPC was missed while no reader was active.
+	*/
+	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);