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Message-Id: <1303126676-3456-2-git-send-email-holler@ahsoftware.de>
Date: Mon, 18 Apr 2011 13:37:56 +0200
From: Alexander Holler <holler@...oftware.de>
To: linux-kernel@...r.kernel.org
Cc: Alexander Holler <holler@...oftware.de>
Subject: [PATCH 1/1] Implement /dev/byte (a generic byte source similiar to /dev/zero)
This device outputs by default 0xff instead 0 which makes more sense
than 0 to clear e.g. FLASH based devices.
To make the device more general usable, the value it outputs is changeable
on a per file descriptor basis through simple writes to it.
Values can be decimal (0 - 255), octal (00 - 0377) or hex (0x0 - 0xff).
For other values (or strings) written to it, the write operation returns an
error and the subsequent output is undefined.
Usage examples:
# Fill /dev/sdX with 0xff.
dd </dev/byte of=/dev/sdX bs=4M
# Create a file of size 10GB and filled with 0xaa.
exec 5<>/dev/byte # Open /dev/byte and assign fd 5 to it
echo 0xaa >&5 # Instruct the device to output 0xaa
dd <&5 of=foo.img bs=10M count=1024 # create and fill the file
exec 5>&- # Close fd 5
Signed-off-by: Alexander Holler <holler@...oftware.de>
---
Documentation/byte.txt | 24 +++++++++++
Documentation/devices.txt | 1 +
drivers/char/Kconfig | 10 ++++
drivers/char/mem.c | 101 +++++++++++++++++++++++++++++++++++++++++++++
4 files changed, 136 insertions(+), 0 deletions(-)
create mode 100644 Documentation/byte.txt
diff --git a/Documentation/byte.txt b/Documentation/byte.txt
new file mode 100644
index 0000000..c132e7d
--- /dev/null
+++ b/Documentation/byte.txt
@@ -0,0 +1,24 @@
+/dev/byte is a generic byte source (similiar to /dev/zero) which uses
+0xff as the default value.
+
+The idea was to have a source for 0xff to erase FLASH based devices like
+SSDs and SD-Cards for which 0xff makes more sense than 0.
+
+To make the device more general usable, the value it outputs is changeable
+on a per file descriptor basis through simple writes to it.
+Values can be decimal (0 - 255), octal (00 - 0377) or hex (0x0 - 0xff).
+For other values (or strings) written to it, the write operation returns an
+error and the subsequent output is undefined.
+
+Usage examples:
+
+# Fill /dev/sdX with 0xff.
+dd </dev/byte of=/dev/sdX bs=4M
+
+# Create a file of size 10GB and filled with 0xaa.
+exec 5<>/dev/byte # Open /dev/byte and assign fd 5 to it
+echo 0xaa >&5 # Instruct the device to output 0xaa
+dd <&5 of=foo.img bs=10M count=1024 # create and fill the file
+exec 5>&- # Close fd 5
+
+Apr 2011, Alexander Holler <holler@...oftware.de>
diff --git a/Documentation/devices.txt b/Documentation/devices.txt
index eccffe7..b34439e 100644
--- a/Documentation/devices.txt
+++ b/Documentation/devices.txt
@@ -101,6 +101,7 @@ Your cooperation is appreciated.
11 = /dev/kmsg Writes to this come out as printk's
12 = /dev/oldmem Used by crashdump kernels to access
the memory of the kernel that crashed.
+ 13 = /dev/byte Generic byte source (default 0xff).
1 block RAM disk
0 = /dev/ram0 First RAM disk
diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index b7980a83..c745bcf 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -97,6 +97,16 @@ config DEVKMEM
kind of kernel debugging operations.
When in doubt, say "N".
+config DEV_BYTE
+ bool "/dev/byte virtual device support"
+ default y
+ help
+ Say Y here if you want to support the /dev/byte device. The
+ /dev/byte device is a generic byte source (similiar to /dev/zero),
+ but uses 0xff as a default value which is changeable on a per
+ file descriptor basis.
+ Read <file:Documentation/byte.txt> for a usage description.
+
config BFIN_JTAG_COMM
tristate "Blackfin JTAG Communication"
depends on BLACKFIN
diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 1256454..7a1d558 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -6,6 +6,7 @@
* Added devfs support.
* Jan-11-1998, C. Scott Ananian <cananian@...mni.princeton.edu>
* Shared /dev/zero mmapping support, Feb 2000, Kanoj Sarcar <kanoj@....com>
+ * Added /dev/byte support, Apr 2011, Alexander Holler <holler@...oftware.de>
*/
#include <linux/mm.h>
@@ -26,6 +27,7 @@
#include <linux/bootmem.h>
#include <linux/splice.h>
#include <linux/pfn.h>
+#include <linux/radix-tree.h>
#include <asm/uaccess.h>
#include <asm/io.h>
@@ -736,6 +738,7 @@ static int open_port(struct inode * inode, struct file * filp)
#define zero_lseek null_lseek
#define full_lseek null_lseek
+#define byte_lseek null_lseek
#define write_zero write_null
#define read_full read_zero
#define open_mem open_port
@@ -835,6 +838,101 @@ static const struct file_operations kmsg_fops = {
.llseek = noop_llseek,
};
+#ifdef CONFIG_DEV_BYTE
+static u16 byte_items[1] = {}; /* must be aligned even */
+static RADIX_TREE(byte_tree, GFP_KERNEL);
+static spinlock_t byte_lock;
+
+/*
+ * Read a value the device should output afterwards.
+ * Values can be decimal (0 - 255), octal (00 - 0377) or hex (0x0 - 0xff).
+ */
+static ssize_t write_byte(struct file *filp, const char __user *buffer,
+ size_t count, loff_t *off)
+{
+ u16 *val;
+ unsigned long num;
+ char buff[5];
+
+ num = (count < sizeof(buff)-1) ? count : sizeof(buff)-1;
+ if (copy_from_user(buff, buffer, num))
+ return -EFAULT;
+ buff[num] = '\0';
+ if (strict_strtoul(buff, 0, &num) || num > 255)
+ return -EFAULT;
+ rcu_read_lock();
+ val = radix_tree_lookup(&byte_tree, (unsigned long)filp);
+ rcu_read_unlock();
+ spin_lock(&byte_lock);
+ if (val != NULL)
+ radix_tree_delete(&byte_tree, (unsigned long)filp);
+ num = radix_tree_insert(&byte_tree, (unsigned long)filp,
+ byte_items+num);
+ spin_unlock(&byte_lock);
+ if (num)
+ return -EFAULT;
+ return count;
+}
+
+static ssize_t read_byte(struct file *file, char __user *buf,
+ size_t count, loff_t *ppos)
+{
+ size_t written;
+ u16 *val;
+ unsigned char byte = 0xff;
+
+ if (!count)
+ return 0;
+
+ if (!access_ok(VERIFY_WRITE, buf, count))
+ return -EFAULT;
+
+ rcu_read_lock();
+ val = radix_tree_lookup(&byte_tree, (unsigned long)file);
+ rcu_read_unlock();
+ if (val != NULL)
+ byte = (unsigned char)(val-byte_items);
+
+ written = 0;
+ while (count) {
+ size_t chunk = count;
+
+ if (chunk > PAGE_SIZE)
+ chunk = PAGE_SIZE; /* Just for latency reasons */
+ memset(buf, byte, chunk);
+ written += chunk;
+ if (signal_pending(current))
+ return written;
+ buf += chunk;
+ count -= chunk;
+ cond_resched();
+ }
+ return written;
+}
+
+static int release_byte(struct inode *inode, struct file *filp)
+{
+ u16 *val;
+
+ rcu_read_lock();
+ val = radix_tree_lookup(&byte_tree, (unsigned long)filp);
+ rcu_read_unlock();
+ if (val != NULL) {
+ spin_lock(&byte_lock);
+ radix_tree_delete(&byte_tree, (unsigned long)filp);
+ spin_unlock(&byte_lock);
+ }
+ return 0;
+}
+
+static const struct file_operations byte_fops = {
+ .llseek = byte_lseek,
+ .read = read_byte,
+ .write = write_byte,
+ .release = release_byte,
+};
+#endif /* CONFIG_DEV_BYTE */
+
static const struct memdev {
const char *name;
mode_t mode;
@@ -857,6 +955,9 @@ static const struct memdev {
#ifdef CONFIG_CRASH_DUMP
[12] = { "oldmem", 0, &oldmem_fops, NULL },
#endif
+#ifdef CONFIG_DEV_BYTE
+ [13] = { "byte", 0666, &byte_fops, NULL },
+#endif
};
static int memory_open(struct inode *inode, struct file *filp)
--
1.7.3.4
--
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