logger: allow starting before the driver is loaded

Don't fail immediately when the driver is not loaded. Use inotify
instead to wait for /sys/kernel/debug/sof/[e]trace to appear.

This makes it possible to start before the driver is loaded which
reduces considerably the chances of missing early logs.

Fixes a small part of https://github.com/thesofproject/linux/issues/3275

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

tools/logger/logger.c

@@ -15,6 +15,12 @@
 #include <fcntl.h>
 #include <stdbool.h>
 #include <termios.h>
+#include <poll.h>
+#include <sys/inotify.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <dirent.h>
+
 #include "convert.h"
 #include "misc.h"
 
@@ -170,6 +176,93 @@ static int append_filter_config(struct convert_config *config, const char *input
 	return 0;
 }
 
+/*
+ * This 5 minutes timeout is for "backward compatible safety" in case
+ * any user/script of an earlier, pre-inotify version of sof-logger
+ * (accidentally?) _expected_ the tool to quit when the driver was not
+ * loaded. This expectation was always wrong but an infinite wait is too
+ * severe of a punishment. This timeout cannot be too small otherwise it
+ * would defeat the new feature.
+ */
+#ifndef SOF_LOGGER_WAIT_OPEN_TIMEOUT_SECS
+#define SOF_LOGGER_WAIT_OPEN_TIMEOUT_SECS (5 * 60)
+#endif
+
+/*
+ * Using inotify, wait up to 5 minutes for 'expected_name' to appear in
+ * 'watched_dir'.  Then return opendir/fopen(expected_name). fopen() and
+ * opendir() failures are NOT handled; e.g. NULL from fopen() is just
+ * passed through.
+ *
+ * Returns a FILE * or DIR *
+ */
+static void *wait_open(const char *watched_dir, const char *expected_file)
+{
+	if (access(watched_dir, R_OK)) {
+		fprintf(stderr, "error: %s() cannot read %s\n", __func__, watched_dir);
+		return NULL;
+	}
+
+	const int iqueue = inotify_init();
+	const int dwatch = inotify_add_watch(iqueue, watched_dir, IN_CREATE);
+	struct stat expected_stat;
+	void *ret_stream = NULL;
+
+	char * const fpath = malloc(strlen(watched_dir) + 1 + strlen(expected_file) + 1);
+
+	strcpy(fpath, watched_dir);
+	strcat(fpath, "/");
+	strcat(fpath, expected_file);
+
+	/* Not racy because the inotify watch was set first. */
+	if (!access(fpath, F_OK))
+		goto fopenit;
+
+	fprintf(stdout, "%s: waiting in %s/ for %s\n", APP_NAME, watched_dir, expected_file);
+	fflush(stdout);
+
+	while (1) {
+		char evlist[1 * sizeof(struct inotify_event)];
+		struct pollfd pfd[1] = {{ .fd = iqueue, .events = POLLIN, .revents = 0 }};
+		int pret = poll(pfd, sizeof(pfd) / sizeof(struct pollfd),
+				SOF_LOGGER_WAIT_OPEN_TIMEOUT_SECS * 1000);
+
+		if (pret == 0) {
+			fprintf(stderr, "error: no %s after waiting %d seconds\n",
+				expected_file, SOF_LOGGER_WAIT_OPEN_TIMEOUT_SECS);
+			goto cleanup;
+		}
+
+		if (pret < 0 || pfd[0].revents != POLLIN) {
+			fprintf(stderr, "error: in %s, poll(%s) returned %d, %d\n",
+				__func__, watched_dir, pret, pfd[0].revents);
+			goto cleanup;
+		}
+
+		/* Silence __attribute__((warn_unused_result)) which is
+		 * enabled by some Linux distros even when broken in gcc:
+		 * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425
+		 */
+		(void)!read(iqueue, evlist, sizeof(evlist));
+
+		if (!access(fpath, F_OK))
+			goto fopenit;
+	}
+
+fopenit:
+	stat(fpath, &expected_stat);
+	if ((expected_stat.st_mode & S_IFMT) == S_IFDIR)
+		ret_stream = opendir(fpath);
+	else
+		ret_stream = fopen(fpath, "rb");
+
+cleanup:
+	free(fpath);
+	inotify_rm_watch(iqueue, dwatch);
+
+	return ret_stream;
+}
+
 int main(int argc, char *argv[])
 {
 	static const char optstring[] = "ho:i:l:ps:c:u:tv:rd:Le:f:gF:n";
@@ -337,7 +430,26 @@ int main(int argc, char *argv[])
 			goto out;
 		}
 	} else if (config.in_file) {
-		config.in_fd = fopen(config.in_file, "rb");
+		static const char sys_debug[] = "/sys/kernel/debug";
+		static const char sys_debug_sof[] = "/sys/kernel/debug/sof";
+		/* In the future we could add an option to force waiting
+		 * for _any_ input file, not just for /sys/kernel/debug/sof/[e]trace
+		 */
+		config.in_fd = NULL;
+		if (strncmp(config.in_file, sys_debug, strlen(sys_debug))) {
+			config.in_fd = fopen(config.in_file, "rb");
+		} else {
+			DIR *dbg_sof = (DIR *)wait_open(sys_debug, "sof");
+
+			if (dbg_sof) {
+				closedir(dbg_sof);
+				config.in_fd =
+					(FILE *)wait_open(sys_debug_sof,
+							  config.in_file +
+							  strlen(sys_debug_sof) + 1);
+			}
+		}
+
 		if (!config.in_fd) {
 			ret = errno;
 			fprintf(stderr,