i2c-pseudo: Fall back on nonseekable_open when stream_open is unavailable.

The FROMLIST revision of i2c-pseudo is targeted at the newest Linux
versions, which have new stream_open function and it is preferred for a
pure I/O stream device.  However updating to the FROMLIST i2c-pseudo
broke compilation with older kernels that lack stream_open.

Since Linux kernel versions as old as 4.18 are still in common use among
Chrome OS developers, this fixes compatibility by checking for
nonseekable_open and stream_open symbols.

nonseekable_open is only used when stream_open is not found and
nonseekable_open is found.  Otherwise stream_open is used.

BRANCH=none
BUG=none
TEST=With Linux 4.18 and Linux 5.2:
$ make
$ # With 4.18 matches nonseekable_open, with 5.2 matches stream_open.
$ strings i2c-pseudo.ko | grep -E -m1 '^(nonseekable_open|stream_open)$'
$ make clean
$ ./install

Signed-off-by: Matthew Blecker <matthewb@chromium.org>
Change-Id: I62c44b1b63e7c4c6a9683be547baa341747f5efa
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2205294
Reviewed-by: Harry Cutts <hcutts@chromium.org>

3 files changed, 46 insertions, 1 deletions

diff --git a/extra/i2c_pseudo/Makefile b/extra/i2c_pseudo/Makefile
index c8dcb1e3b4..f7fda6e2de 100644
--- a/extra/i2c_pseudo/Makefile
+++ b/extra/i2c_pseudo/Makefile
@@ -1,9 +1,20 @@
+# Copyright 2020 The Chromium OS Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+#
+# Makefile for i2c-pseudo module.  Typical usage:
+#   $ make
+#   $ sudo make modules_install
+#   $ make clean
+
 obj-m := i2c-pseudo.o
 
 .PHONY: all
 
 all: modules
 
+CFLAGS_i2c-pseudo.o += "-DHAVE_STREAM_OPEN=$(shell "$M"/check_stream_open.sh)"
+
 .DEFAULT:
 	$(MAKE) -C /lib/modules/$(shell uname -r)/build \
 		M=$(shell pwd) \
diff --git a/extra/i2c_pseudo/check_stream_open.sh b/extra/i2c_pseudo/check_stream_open.sh
new file mode 100755
index 0000000000..da802cb282
--- /dev/null
+++ b/extra/i2c_pseudo/check_stream_open.sh
@@ -0,0 +1,23 @@
+#!/bin/sh
+#
+# Copyright 2020 The Chromium OS Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+#
+# This checks whether stream_open symbol is available from the target kernel.
+#
+# Output meanings:
+#   -1 : stream_open is not available
+#    0 : unknown whether or not stream_open is available
+#    1 : stream_open is available
+
+symbols="$(< "/lib/modules/$(uname -r)/build/Module.symvers" \
+           awk '{print $2}' | grep -E '^(nonseekable_open|stream_open)$')"
+
+if echo "${symbols}" | grep -q '^stream_open$'; then
+	echo 1
+elif echo "${symbols}" | grep -q '^nonseekable_open$'; then
+	echo -1
+else
+	echo 0
+fi
diff --git a/extra/i2c_pseudo/i2c-pseudo.c b/extra/i2c_pseudo/i2c-pseudo.c
index 69fc9dbc52..bbe702a14c 100644
--- a/extra/i2c_pseudo/i2c-pseudo.c
+++ b/extra/i2c_pseudo/i2c-pseudo.c
@@ -2205,8 +2205,19 @@ static int i2cp_cdev_open(struct inode *inodep, struct file *filep)
 	/* Is there any way to find this through @inodep? */
 	this_pseudo = i2cp_device;
 
-	/* I2C pseudo adapter controllers are not seekable. */
+	/*
+	 * HAVE_STREAM_OPEN value meanings:
+	 *   -1 : stream_open() is not available
+	 *    0 : unknown if stream_open() is or is not available
+	 *    1 : stream_open() is available
+	 */
+#if HAVE_STREAM_OPEN >= 0
+	/* I2C pseudo adapter controllers are non-seekable pure I/O streams. */
 	stream_open(inodep, filep);
+#else
+	/* I2C pseudo adapter controllers are not seekable. */
+	nonseekable_open(inodep, filep);
+#endif
 	/* Refuse fsnotify events.  Modeled after /dev/ptmx implementation. */
 	filep->f_mode |= FMODE_NONOTIFY;