[libgpiod][PATCH] build: allow building with pre-v5.5 kernel headers

[Bartosz Golaszewski <brgl@bgdev.pl>]

From: Bartosz Golaszewski <bgolaszewski@baylibre.com>

libgpiod v1.6 requires at least v5.5 linux kernel headers to build. This
is because several new symbols have been defined in linux v5.5. In order
to allow building with kernel headers v4.6 and on, let's check the
presence of the new symbols and redefine them if needed.

The new features won't work on kernels pre v5.5 but the build won't fail
anymore.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Michael Nosthoff <buildroot@heine.tech>
---
Downstream complains a lot about libgpiod v1.6 not building with kernel
headers pre-v5.5. We've had a discussion on that over at buildroot[1].

For libgpiod v2.0 we need to figure out a better way of dealing with new
kernel features added after the release of v2.0 but for v1.6 let's just
redefine the symbols if needed.

[1] http://lists.busybox.net/pipermail/buildroot/2020-October/295314.html

 configure.ac | 10 +++++++++-
 lib/core.c   | 18 ++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index cd337ff..6aef289 100644
--- a/configure.ac
+++ b/configure.ac
@@ -95,13 +95,21 @@ AC_CHECK_HEADERS([sys/sysmacros.h], [], [HEADER_NOT_FOUND_LIB([sys/sysmacros.h])
 AC_CHECK_HEADERS([linux/gpio.h], [], [HEADER_NOT_FOUND_LIB([linux/gpio.h])])
 AC_CHECK_HEADERS([linux/version.h], [], [HEADER_NOT_FOUND_LIB([linux/version.h])])
 
+AC_COMPILE_IFELSE([AC_LANG_SOURCE(
+#include <linux/version.h>
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 6, 0)
+#error
+#endif
+)],
+[], [AC_MSG_ERROR(["libgpiod needs linux headers version >= v4.6.0"])])
+
 AC_COMPILE_IFELSE([AC_LANG_SOURCE(
 #include <linux/version.h>
 #if LINUX_VERSION_CODE < KERNEL_VERSION(5, 5, 0)
 #error
 #endif
 )],
-[], [AC_MSG_ERROR(["libgpiod needs linux headers version >= v5.5.0"])])
+[], [AC_DEFINE([KERNEL_PRE_5_5], [], [We need to define symbols added in linux v5.5])])
 
 AC_ARG_ENABLE([tools],
 	[AC_HELP_STRING([--enable-tools],
diff --git a/lib/core.c b/lib/core.c
index b964272..f49743b 100644
--- a/lib/core.c
+++ b/lib/core.c
@@ -22,6 +22,24 @@
 #include <sys/types.h>
 #include <unistd.h>
 
+#ifdef KERNEL_PRE_5_5
+#define GPIOLINE_FLAG_BIAS_PULL_UP		(1UL << 5)
+#define GPIOLINE_FLAG_BIAS_PULL_DOWN		(1UL << 6)
+#define GPIOLINE_FLAG_BIAS_DISABLE		(1UL << 7)
+
+#define GPIOHANDLE_REQUEST_BIAS_PULL_UP		(1UL << 5)
+#define GPIOHANDLE_REQUEST_BIAS_PULL_DOWN	(1UL << 6)
+#define GPIOHANDLE_REQUEST_BIAS_DISABLE		(1UL << 7)
+
+struct gpiohandle_config {
+	__u32 flags;
+	__u8 default_values[GPIOHANDLES_MAX];
+	__u32 padding[4]; /* padding for future use */
+};
+
+#define GPIOHANDLE_SET_CONFIG_IOCTL _IOWR(0xB4, 0x0a, struct gpiohandle_config)
+#endif /* KERNEL_PRE_5_5 */
+
 enum {
 	LINE_FREE = 0,
 	LINE_REQUESTED_VALUES,
-- 
2.29.1