media: v4l2-compat-ioctl32: better document the code

This file does a lot of non-trivial struff. Document it using
kernel-doc markups where needed and improve the comments inside
do_video_ioctl().
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/v4l2-core/v4l2-compat-ioctl32.c

@@ -22,7 +22,18 @@
 #include <media/v4l2-ctrls.h>
 #include <media/v4l2-ioctl.h>
 
-/* Use the same argument order as copy_in_user */
+/**
+ * assign_in_user() - Copy from one __user var to another one
+ *
+ * @to: __user var where data will be stored
+ * @from: __user var where data will be retrieved.
+ *
+ * As this code very often needs to allocate userspace memory, it is easier
+ * to have a macro that will do both get_user() and put_user() at once.
+ *
+ * This function complements the macros defined at asm-generic/uaccess.h.
+ * It uses the same argument order as copy_in_user()
+ */
 #define assign_in_user(to, from)					\
 ({									\
 	typeof(*from) __assign_tmp;					\
@@ -30,16 +41,56 @@
 	get_user(__assign_tmp, from) || put_user(__assign_tmp, to);	\
 })
 
+/**
+ * get_user_cast() - Stores at a kernelspace local var the contents from a
+ *		pointer with userspace data that is not tagged with __user.
+ *
+ * @__x: var where data will be stored
+ * @__ptr: var where data will be retrieved.
+ *
+ * Sometimes we need to declare a pointer without __user because it
+ * comes from a pointer struct field that will be retrieved from userspace
+ * by the 64-bit native ioctl handler. This function ensures that the
+ * @__ptr will be cast to __user before calling get_user() in order to
+ * avoid warnings with static code analyzers like smatch.
+ */
 #define get_user_cast(__x, __ptr)					\
 ({									\
 	get_user(__x, (typeof(*__ptr) __user *)(__ptr));		\
 })
 
+/**
+ * put_user_force() - Stores the contents of a kernelspace local var
+ *		      into a userspace pointer, removing any __user cast.
+ *
+ * @__x: var where data will be stored
+ * @__ptr: var where data will be retrieved.
+ *
+ * Sometimes we need to remove the __user attribute from some data,
+ * by passing the __force macro. This function ensures that the
+ * @__ptr will be cast with __force before calling put_user(), in order to
+ * avoid warnings with static code analyzers like smatch.
+ */
 #define put_user_force(__x, __ptr)					\
 ({									\
 	put_user((typeof(*__x) __force *)(__x), __ptr);			\
 })
 
+/**
+ * assign_in_user_cast() - Copy from one __user var to another one
+ *
+ * @to: __user var where data will be stored
+ * @from: var where data will be retrieved that needs to be cast to __user.
+ *
+ * As this code very often needs to allocate userspace memory, it is easier
+ * to have a macro that will do both get_user_cast() and put_user() at once.
+ *
+ * This function should be used instead of assign_in_user() when the @from
+ * variable was not declared as __user. See get_user_cast() for more details.
+ *
+ * This function complements the macros defined at asm-generic/uaccess.h.
+ * It uses the same argument order as copy_in_user()
+ */
 #define assign_in_user_cast(to, from)					\
 ({									\
 	typeof(*from) __assign_tmp;					\
@@ -47,7 +98,16 @@
 	get_user_cast(__assign_tmp, from) || put_user(__assign_tmp, to);\
 })
 
-
+/**
+ * native_ioctl - Ancillary function that calls the native 64 bits ioctl
+ * handler.
+ *
+ * @file: pointer to &struct file with the file handler
+ * @cmd: ioctl to be called
+ * @arg: arguments passed from/to the ioctl handler
+ *
+ * This function calls the native ioctl handler at v4l2-dev, e. g. v4l2_ioctl()
+ */
 static long native_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {
 	long ret = -ENOIOCTLCMD;
@@ -59,6 +119,21 @@ static long native_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 }
 
 
+/*
+ * Per-ioctl data copy handlers.
+ *
+ * Those come in pairs, with a get_v4l2_foo() and a put_v4l2_foo() routine,
+ * where "v4l2_foo" is the name of the V4L2 struct.
+ *
+ * They basically get two __user pointers, one with a 32-bits struct that
+ * came from the userspace call and a 64-bits struct, also allocated as
+ * userspace, but filled internally by do_video_ioctl().
+ *
+ * For ioctls that have pointers inside it, the functions will also
+ * receive an ancillary buffer with extra space, used to pass extra
+ * data to the routine.
+ */
+
 struct v4l2_clip32 {
 	struct v4l2_rect        c;
 	compat_caddr_t		next;
@@ -1010,6 +1085,13 @@ static int put_v4l2_edid32(struct v4l2_edid __user *p64,
 	return 0;
 }
 
+/*
+ * List of ioctls that require 32-bits/64-bits conversion
+ *
+ * The V4L2 ioctls that aren't listed there don't have pointer arguments
+ * and the struct size is identical for both 32 and 64 bits versions, so
+ * they don't need translations.
+ */
 
 #define VIDIOC_G_FMT32		_IOWR('V',  4, struct v4l2_format32)
 #define VIDIOC_S_FMT32		_IOWR('V',  5, struct v4l2_format32)
@@ -1038,6 +1120,21 @@ static int put_v4l2_edid32(struct v4l2_edid __user *p64,
 #define VIDIOC_G_OUTPUT32	_IOR ('V', 46, s32)
 #define VIDIOC_S_OUTPUT32	_IOWR('V', 47, s32)
 
+/**
+ * alloc_userspace() - Allocates a 64-bits userspace pointer compatible
+ *	for calling the native 64-bits version of an ioctl.
+ *
+ * @size:	size of the structure itself to be allocated.
+ * @aux_space:	extra size needed to store "extra" data, e.g. space for
+ *		other __user data that is pointed to fields inside the
+ *		structure.
+ * @new_p64:	pointer to a pointer to be filled with the allocated struct.
+ *
+ * Return:
+ *
+ * if it can't allocate memory, either -ENOMEM or -EFAULT will be returned.
+ * Zero otherwise.
+ */
 static int alloc_userspace(unsigned int size, u32 aux_space,
 			   void __user **new_p64)
 {
@@ -1049,6 +1146,23 @@ static int alloc_userspace(unsigned int size, u32 aux_space,
 	return 0;
 }
 
+/**
+ * do_video_ioctl() - Ancillary function with handles a compat32 ioctl call
+ *
+ * @file: pointer to &struct file with the file handler
+ * @cmd: ioctl to be called
+ * @arg: arguments passed from/to the ioctl handler
+ *
+ * This function is called when a 32 bits application calls a V4L2 ioctl
+ * and the Kernel is compiled with 64 bits.
+ *
+ * This function is called by v4l2_compat_ioctl32() when the function is
+ * not private to some specific driver.
+ *
+ * It converts a 32-bits struct into a 64 bits one, calls the native 64-bits
+ * ioctl handler and fills back the 32-bits struct with the results of the
+ * native call.
+ */
 static long do_video_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {
 	void __user *p32 = compat_ptr(arg);
@@ -1058,7 +1172,9 @@ static long do_video_ioctl(struct file *file, unsigned int cmd, unsigned long ar
 	int compatible_arg = 1;
 	long err = 0;
 
-	/* First, convert the command. */
+	/*
+	 * 1. When struct size is different, converts the command.
+	 */
 	switch (cmd) {
 	case VIDIOC_G_FMT32: cmd = VIDIOC_G_FMT; break;
 	case VIDIOC_S_FMT32: cmd = VIDIOC_S_FMT; break;
@@ -1087,6 +1203,11 @@ static long do_video_ioctl(struct file *file, unsigned int cmd, unsigned long ar
 	case VIDIOC_S_EDID32: cmd = VIDIOC_S_EDID; break;
 	}
 
+	/*
+	 * 2. Allocates a 64-bits userspace pointer to store the
+	 * values of the ioctl and copy data from the 32-bits __user
+	 * argument into it.
+	 */
 	switch (cmd) {
 	case VIDIOC_OVERLAY:
 	case VIDIOC_STREAMON:
@@ -1209,6 +1330,15 @@ static long do_video_ioctl(struct file *file, unsigned int cmd, unsigned long ar
 	if (err)
 		return err;
 
+	/*
+	 * 3. Calls the native 64-bits ioctl handler.
+	 *
+	 * For the functions where a conversion was not needed,
+	 * compatible_arg is true, and it will call it with the arguments
+	 * provided by userspace and stored at @p32 var.
+	 *
+	 * Otherwise, it will pass the newly allocated @new_p64 argument.
+	 */
 	if (compatible_arg)
 		err = native_ioctl(file, cmd, (unsigned long)p32);
 	else
@@ -1218,9 +1348,14 @@ static long do_video_ioctl(struct file *file, unsigned int cmd, unsigned long ar
 		return err;
 
 	/*
-	 * Special case: even after an error we need to put the
-	 * results back for these ioctls since the error_idx will
-	 * contain information on which control failed.
+	 * 4. Special case: even after an error we need to put the
+	 * results back for some ioctls.
+	 *
+	 * In the case of EXT_CTRLS, the error_idx will contain information
+	 * on which control failed.
+	 *
+	 * In the case of S_EDID, the driver can return E2BIG and set
+	 * the blocks to maximum allowed value.
 	 */
 	switch (cmd) {
 	case VIDIOC_G_EXT_CTRLS:
@@ -1237,6 +1372,10 @@ static long do_video_ioctl(struct file *file, unsigned int cmd, unsigned long ar
 	if (err)
 		return err;
 
+	/*
+	 * 5. Copy the data returned at the 64 bits userspace pointer to
+	 * the original 32 bits structure.
+	 */
 	switch (cmd) {
 	case VIDIOC_S_INPUT:
 	case VIDIOC_S_OUTPUT:
@@ -1287,6 +1426,20 @@ static long do_video_ioctl(struct file *file, unsigned int cmd, unsigned long ar
 	return err;
 }
 
+/**
+ * v4l2_compat_ioctl32() - Handles a compat32 ioctl call
+ *
+ * @file: pointer to &struct file with the file handler
+ * @cmd: ioctl to be called
+ * @arg: arguments passed from/to the ioctl handler
+ *
+ * This function is meant to be used as .compat_ioctl fops at v4l2-dev.c
+ * in order to deal with 32-bit calls on a 64-bits Kernel.
+ *
+ * This function calls do_video_ioctl() for non-private V4L2 ioctls.
+ * If the function is a private one it calls vdev->fops->compat_ioctl32
+ * instead.
+ */
 long v4l2_compat_ioctl32(struct file *file, unsigned int cmd, unsigned long arg)
 {
 	struct video_device *vdev = video_devdata(file);