lib: add igt_debugfs_read()

A helpful function for when you want to read a whole debugfs file to a
string and don't want to worry about opening and closing file
descriptors and asserting buffer sizes.

We've been using this already for kms_frontbuffer_tracking and
kms_fbcon_fbt, so the only test with new code here is kms_fbc_crc.

Also notice that for kms_fbc_crc we had to increase the buffer size
since the file can sometimes be bigger than 64 bytes - depending on
the reason why FBC is disabled.

Of course, there are probably many other programs we can patch, but
I'm not doing this now.

v2: Add the macro to wrap sizeof() (Daniel).
v3: Add documentation for the macro too (Daniel).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>

lib/igt_debugfs.c

@@ -185,6 +185,33 @@ FILE *igt_debugfs_fopen(const char *filename,
 	return fopen(buf, mode);
 }
 
+/**
+ * __igt_debugfs_read:
+ * @filename: file name
+ * @buf: buffer where the contents will be stored, allocated by the caller
+ * @buf_size: size of the buffer
+ *
+ * This function opens the debugfs file, reads it, stores the content in the
+ * provided buffer, then closes the file. Users should make sure that the buffer
+ * provided is big enough to fit the whole file, plus one byte.
+ */
+void __igt_debugfs_read(const char *filename, char *buf, int buf_size)
+{
+	FILE *file;
+	size_t n_read;
+
+	file = igt_debugfs_fopen(filename, "r");
+	igt_assert(file);
+
+	n_read = fread(buf, 1, buf_size - 1, file);
+	igt_assert(n_read > 0);
+	igt_assert(feof(file));
+
+	buf[n_read] = '\0';
+
+	igt_assert(fclose(file) == 0);
+}
+
 /*
  * Pipe CRC
  */

lib/igt_debugfs.h

@@ -34,6 +34,18 @@ enum pipe;
 int igt_debugfs_open(const char *filename, int mode);
 FILE *igt_debugfs_fopen(const char *filename,
 			const char *mode);
+void __igt_debugfs_read(const char *filename, char *buf, int buf_size);
+
+/**
+ * igt_debugfs_read:
+ * @filename: name of the debugfs file
+ * @buf: buffer where the contents will be stored, allocated by the caller.
+ *
+ * This is just a convenience wrapper for __igt_debugfs_read. See its
+ * documentation.
+ */
+#define igt_debugfs_read(filename, buf) \
+		__igt_debugfs_read((filename), (buf), sizeof(buf))
 
 /*
  * Pipe CRC