seq_file: move doc to README

README.adoc

@@ -2976,7 +2976,7 @@ This example shows how sysfs is more restricted, as it does not take an arbitrar
 
 So you basically can only do `open`, `close`, `read`, `write`, and `lseek` on sysfs files.
 
-It is similar to a `seq_file` file operation, except that write is also implemented.
+It is similar to a <<seq_file>> file operation, except that write is also implemented.
 
 TODO: what are those `kobject` structs? Make a more complex example that shows what they can do.
 
@@ -3024,6 +3024,76 @@ File operations is the main method of userland driver communication.
 
 No, there no official documentation: http://stackoverflow.com/questions/15213932/what-are-the-struct-file-operations-arguments
 
+==== seq_file
+
+In guest:
+
+....
+/seq_file.sh
+echo $?
+....
+
+Outcome: the test passes:
+
+....
+0
+....
+
+Sources:
+
+* link:kernel_module/seq_file.c[]
+* link:rootfs_overlay/seq_file.sh[]
+
+Writing trivial read <<file-operations>> is repetitive and error prone.
+
+The `seq_file` API makes the process much easier for those trivial cases.
+
+In this example we create a debugfs file that behaves just like a file that contains:
+
+....
+0
+1
+2
+....
+
+However, we only store a single integer in memory and calculate the file on the fly in an iterator fashion.
+
+`seq_file` does not provide `write`: https://stackoverflow.com/questions/30710517/how-to-implement-a-writable-proc-file-by-using-seq-file-in-a-driver-module
+
+Bibliography:
+
+* link:https://github.com/torvalds/linux/blob/v4.17/Documentation/filesystems/seq_file.txt[Documentation/filesystems/seq_file.txt]
+* https://stackoverflow.com/questions/25399112/how-to-use-a-seq-file-in-linux-modules
+
+===== seq_file single_open
+
+In guest:
+
+....
+/seq_file.sh
+echo $?
+....
+
+Outcome: the test passes:
+
+....
+0
+....
+
+Sources:
+
+* link:kernel_module/seq_file_single_open.c[]
+* link:rootfs_overlay/seq_file_single_open.sh[]
+
+If you have the entire read output upfront, `single_open` is an even more convenient version of <<seq_file>>.
+
+This example produces a debugfs file that behaves like a file that contains:
+
+....
+ab
+cd
+....
+
 ==== Character devices
 
 In guest:
@@ -3123,10 +3193,9 @@ Sources:
 * link:kernel_module/user/anonymous_inode.c[]
 * link:rootfs_overlay/anonymous_inode.sh[]
 
-This example:
+This example gets an anonymous inode via `ioctl` from a debugfs entry by using `anon_inode_getfd`.
 
-* gets an anonymous inode via `ioctl` from a debugfs entry `anon_inode_getfd` from a debugfs file
+Reads to that inode return the sequence: `1`, `10`, `100`, ... `10000000`, `1`, `100`, ...
-* read jiffies from that inode
 
 Anonymous inodes allow getting multiple file descriptors from a single filesystem entry, which reduces namespace pollution compared to creating multiple device files.
 

kernel_module/README.adoc

@@ -22,8 +22,6 @@
 .. link:ioctl.c[]
 .. link:mmap.c[]
 .. link:poll.c[]
-.. link:seq_file.c[]
-.. link:seq_file_inode.c[]
 . Asynchronous
 .. link:irq.c[]
 .. link:kthread.c[]

kernel_module/anonymous_inode.c

@@ -24,6 +24,9 @@ static ssize_t read(struct file *filp, char __user *buf, size_t len, loff_t *off
 		ret = -EFAULT;
 	}
 	myval <<= 4;
+	if (myval == 0) {
+		myval = 1;
+	}
 	return ret;
 }
 

kernel_module/seq_file.c

@@ -1,28 +1,4 @@
-/*
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#seq_file */
-Adapted from: Documentation/filesystems/seq_file.txt
-but we limit the count to the max module parameter.
-
-Writting trivial read fops is repetitive and error prone.
-
-The seq_file API makes the process much easier for those trivial cases.
-
-This example is behaves just like a file that contains:
-
-	0
-	1
-	2
-
-However, we only store a single integer in memory
-and calculate the file on the fly in an iterator fashion.
-
-There is not write version, as writes are more complex:
-https://stackoverflow.com/questions/30710517/how-to-implement-a-writable-proc-file-by-using-seq-file-in-a-driver-module
-
-Bibliography:
-
-- Documentation/filesystems/seq_file.txt
-- https://stackoverflow.com/questions/25399112/how-to-use-a-seq-file-in-linux-modules
-*/
 
 #include <linux/debugfs.h>
 #include <linux/errno.h> /* EFAULT */

kernel_module/seq_file_single_open.c

@@ -1,12 +1,4 @@
-/*
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#seq_file-single_open */
-If you have the entire read output upfront, single_open
-is an even more convenient version of seq_file.
-
-This example behaves like a file that contains:
-
-	ab
-	cd
-*/
 
 #include <linux/debugfs.h>
 #include <linux/errno.h> /* EFAULT */
@@ -40,8 +32,7 @@ static const struct file_operations fops = {
 
 static int myinit(void)
 {
-	debugfs_file = debugfs_create_file(
+	debugfs_file = debugfs_create_file("lkmc_seq_file_single_open", S_IRUSR, NULL, NULL, &fops);
-		"lkmc_seq_file_single", S_IRUSR, NULL, NULL, &fops);
 	return 0;
 }
 

rootfs_overlay/seq_file.sh

@@ -1,29 +1,10 @@
 #!/bin/sh
-
+set -e
-set -ex
+f=/sys/kernel/debug/lkmc_seq_file
-
 insmod /seq_file.ko
-cd /sys/kernel/debug
+[ "$(cat "$f")" = "$(printf '0\n1\n2\n')" ]
-
+[ "$(cat "$f")" = "$(printf '0\n1\n2\n')" ]
-cat 'lkmc_seq_file'
+[ "$(dd if="$f" bs=1 count=2 skip=0 status=none)" = "$(printf '0\n')" ]
-# => 0
+[ "$(dd if="$f" bs=1 count=2 skip=2 status=none)" = "$(printf '1\n')" ]
-# => 1
+[ "$(dd if="$f" bs=4 count=1 skip=0 status=none)" = "$(printf '0\n1\n')" ]
-# => 2
-
-cat 'lkmc_seq_file'
-# => 0
-# => 1
-# => 2
-
-dd if='lkmc_seq_file' bs=1 count=2 skip=0 status=none
-# => 0
-dd if='lkmc_seq_file' bs=1 count=4 skip=0 status=none
-# => 0
-# => 1
-dd if='lkmc_seq_file' bs=1 count=2 skip=2 status=none
-# => 1
-dd if='lkmc_seq_file' bs=4 count=1 skip=0 status=none
-# => 0
-# => 1
-
 rmmod seq_file

rootfs_overlay/seq_file_single.sh

@@ -1,12 +0,0 @@
-#!/bin/sh
-set -ex
-insmod /seq_file_single.ko
-cd /sys/kernel/debug
-cat 'lkmc_seq_file_single'
-# => ab
-# => cd
-dd if='lkmc_seq_file_single' bs=1 count=3 skip=1
-# => b
-# => c
-
-rmmod seq_file_single

rootfs_overlay/seq_file_single_open.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+set -e
+f=/sys/kernel/debug/lkmc_seq_file_single_open
+insmod /seq_file_single_open.ko
+[ "$(cat "$f")" = "$(printf 'ab\ncd\n')" ]
+[ "$(dd if="$f" bs=1 count=3 skip=1)" = "$(printf "b\nc\n")" ]
+rmmod seq_file_single_open

rootfs_overlay/test_all.sh

@@ -6,8 +6,10 @@ for test in \
   /debugfs.sh \
   /fops.sh \
   /procfs.sh \
-  /sysfs.sh
+  /seq_file.sh \
-do
+  /seq_file_single_open.sh \
+  /sysfs.sh \
+; do
   if ! "$test"; then
     echo "lkmc_test_fail: ${test}"
     exit 1