From 04465b1a44c25bf2352f0073d5e85577e7d1b817 Mon Sep 17 00:00:00 2001
From: asas1asas200 <asas1asas200@gmail.com>
Date: Sun, 17 Apr 2022 22:20:18 +0800
Subject: [PATCH] Add description of sysfs attribute
The description of the attribute was added in sysfs section and referenced in vinput section.
In vinput section, just described the class_attribute and some macros about sysfs class.
Add file name at vinput-related examples begin.
---
examples/vinput.c | 4 ++++
examples/vinput.h | 4 ++++
examples/vkbd.c | 4 ++++
lkmpg.tex | 58 ++++++++++++++++++++++++++++++++++++++++++++++-
4 files changed, 69 insertions(+), 1 deletion(-)
diff --git a/examples/vinput.c b/examples/vinput.c
index 75e2a76..b692d3f 100644
--- a/examples/vinput.c
+++ b/examples/vinput.c
@@ -1,3 +1,7 @@
+/*
+ * vinput.c
+ */
+
#include <linux/cdev.h>
#include <linux/input.h>
#include <linux/module.h>
diff --git a/examples/vinput.h b/examples/vinput.h
index 394b061..c77c138 100644
--- a/examples/vinput.h
+++ b/examples/vinput.h
@@ -1,3 +1,7 @@
+/*
+ * vinput.h
+ */
+
#ifndef VINPUT_H
#define VINPUT_H
diff --git a/examples/vkbd.c b/examples/vkbd.c
index 7b1a6a0..d1cb833 100644
--- a/examples/vkbd.c
+++ b/examples/vkbd.c
@@ -1,3 +1,7 @@
+/*
+ * vkbd.c
+ */
+
#include <linux/init.h>
#include <linux/input.h>
#include <linux/module.h>
diff --git a/lkmpg.tex b/lkmpg.tex
index df9c060..7af09ca 100644
--- a/lkmpg.tex
+++ b/lkmpg.tex
@@ -1257,6 +1257,40 @@ You can find sysfs directories and files under the \verb|/sys| directory on your
ls -l /sys
\end{codebash}
+Attributes can be exported for kobjects in the form of regular files in the filesystem.
+Sysfs forwards file I/O operations to methods defined for the attributes, providing a means to read and write kernel attributes.
+
+An attribute definition in simply:
+
+\begin{code}
+struct attribute {
+ char *name;
+ struct module *owner;
+ umode_t mode;
+};
+
+int sysfs_create_file(struct kobject * kobj, const struct attribute * attr);
+void sysfs_remove_file(struct kobject * kobj, const struct attribute * attr);
+\end{code}
+
+For example, the driver model defines \cpp|struct device_attribute| like:
+
+\begin{code}
+struct device_attribute {
+ struct attribute attr;
+ ssize_t (*show)(struct device *dev, struct device_attribute *attr,
+ char *buf);
+ ssize_t (*store)(struct device *dev, struct device_attribute *attr,
+ const char *buf, size_t count);
+};
+
+int device_create_file(struct device *, const struct device_attribute *);
+void device_remove_file(struct device *, const struct device_attribute *);
+\end{code}
+
+To read or write attributes, \cpp|show()| or \cpp|store()| method must be specified when declaring the attribute.
+For the common cases \src{include/linux/sysfs.h} provides convenience macros (\cpp|__ATTR|, \cpp|__ATTR_RO|, \cpp|__ATTR_WO|, etc.) to make defining attributes easier as well as making code more concise and readable.
+
An example of a hello world module which includes the creation of a variable accessible via sysfs is given below.
\samplec{examples/hello-sysfs.c}
@@ -1293,6 +1327,12 @@ Finally, remove the test module:
sudo rmmod hello_sysfs
\end{codebash}
+In the above case, we use a simple kobject to create a directory under sysfs, and communicate with its attributes.
+Since Linux v2.6.0, the \cpp|kobject| structure made its appearance.
+It was initially meant as a simple way of unifying kernel code which manages reference counted objects.
+After a bit of mission creep, it is now the glue that holds much of the device model and its sysfs interface together.
+For more information about kobject and sysfs, see \src{Documentation/driver-api/driver-model/driver.rst} and \url{https://lwn.net/Articles/51437/}.
+
\section{Talking To Device Files}
\label{sec:device_files}
Device files are supposed to represent physical devices.
@@ -1897,6 +1937,23 @@ The buffer will then be copied to user.
vinput devices are created and destroyed using sysfs.
And, event injection is done through a \verb|/dev| node.
The device name will be used by the userland to export a new virtual input device.
+
+The \cpp|class_attribute| structure is similar to other attribute types we talked about in section \ref{sec:sysfs}:
+
+\begin{code}
+struct class_attribute {
+ struct attribute attr;
+ ssize_t (*show)(struct class *class, struct class_attribute *attr,
+ char *buf);
+ ssize_t (*store)(struct class *class, struct class_attribute *attr,
+ const char *buf, size_t count);
+};
+\end{code}
+
+In \verb|vinput.c|, the macro \cpp|CLASS_ATTR_WO(export/unexport)| defined in \src{include/linux/device.h} (in this case, \verb|device.h| is included in \src{include/linux/input.h}) will generate the \cpp|class_attribute| structures which are named \verb|class_attr_export/unexport|.
+Then, put them into \cpp|vinput_class_attrs| array and the macro \cpp|ATTRIBUTE_GROUPS(vinput_class)| will generate the \cpp|struct attribute_group vinput_class_group| that should be assigned in \cpp|vinput_class|.
+Finally, call \cpp|class_register(&vinput_class)| to create attributes in sysfs.
+
To create a \verb|vinputX| sysfs entry and \verb|/dev| node.
\begin{codebash}
@@ -1933,7 +1990,6 @@ echo "-34" | sudo tee /dev/vinput0
\samplec{examples/vkbd.c}
-% TODO: Add description of attribute
% TODO: Add vts.c and vmouse.c example
\section{Standardizing the interfaces: The Device Model}