ASoC: SOF: llext_load: fix single-shot write and add noop_llseek

The sof_llext_dfs_write handler requires the complete library binary
delivered in a single write() syscall (it returns -EINVAL if *ppos != 0
on entry).  The 'cat' command splits files larger than ~16 KB into
BUFSIZ-sized chunks, so the first chunk triggered a truncated IPC4
LOAD_LIBRARY with only a partial payload, causing the DSP DMA to time
out waiting for the remaining bytes.

Two fixes:
1. Add .llseek = noop_llseek to sof_llext_load_fops so that tools like
   'dd' that call lseek(SEEK_CUR) to determine the current position do
   not get ESPIPE and abort before writing anything.

2. Improve the -EINVAL error message when a non-zero *ppos write is
   rejected to tell the user to use 'dd bs=<size> count=1' instead of
   'cat'.

Users and scripts should write the module binary with:
  dd if=<module.ri> of=/sys/kernel/debug/sof/llext_load \
     bs=$(stat -c%s <module.ri>) count=1

Signed-off-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>

Author: lrgirdwo

sound/soc/sof/sof-client-llext-load.c

@@ -77,8 +77,13 @@ static ssize_t sof_llext_dfs_write(struct file *file,
 	u32 lib_id, state;
 	int ret;
 
-	if (*ppos)
+	if (*ppos) {
+		dev_err_ratelimited(&cdev->auxdev.dev,
+			"llext_load: partial write rejected (offset=%lld) — "
+			"use 'dd if=<module.ri> of=/sys/kernel/debug/sof/llext_load bs=$(stat -c%%s <module.ri>) count=1'\n",
+			*ppos);
 		return -EINVAL;		/* no seek; single-shot write only */
+	}
 
 	if (!count || count > SOF_LLEXT_MAX_SIZE)
 		return count ? -EFBIG : -EINVAL;
@@ -217,6 +222,7 @@ static const struct file_operations sof_llext_load_fops = {
 	.open    = sof_llext_dfs_open,
 	.write   = sof_llext_dfs_write,
 	.release = sof_llext_dfs_release,
+	.llseek  = noop_llseek,	/* allow lseek(0) from dd/tools; write must start at pos=0 */
 	.owner   = THIS_MODULE,
 };