The tty/serial core uses 5 bits in the tty_port.flags field to
manage state. They are:
ASYNCB_INITIALIZED
ASYNCB_SUSPENDED
ASYNCB_NORMAL_ACTIVE
- and -
ASYNCB_CTS_FLOW
ASYNCB_CHECK_CD
Unfortunately, updates to this field (tty_port.flags) are often
non-atomic. Additionally, the field is visible to/modifiable by
userspace (the first 3 bits above are not modifiable by userspace
though). Lastly, the multi-bit ASYNC_SPD_ bitfield is in this
tty_port.flags field as well.
What needs to happen is the tty/serial core needs to update its
state transitions atomically. I want to re-define the above 5 flags
into a separate field in the tty_port structure and designate new
symbols for these bits. The base patch that does this is inlined
below.
This will break out-of-tree drivers but I don't really see a
realistic alternative. Also, I think the new symbol prefix ASY_ isn't
great and I'd like to get some suggestions.
Regards,
Peter Hurley
--- >% ---
Subject: [PATCH] tty: Define ASYNC_ replacement bits
Prepare for relocating kernel private state bits out of tty_port::flags
field; tty_port::flags field is not atomic and can become corrupted
by concurrent updates. It also suffers from the complication of sharing
in a userspace-visible field which must be masked.
Define new tty_port::iflags field and new, substitute bit definitions
for the former ASYNC_* flags.
---
include/linux/tty.h | 16 +++++++++++++++-
include/uapi/linux/tty_flags.h | 9 ++++++++-
2 files changed, 23 insertions(+), 2 deletions(-)
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 3b09f23..4170eed 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -228,7 +228,8 @@ struct tty_port {
int count; /* Usage count */
wait_queue_head_t open_wait; /* Open waiters */
wait_queue_head_t delta_msr_wait; /* Modem status change */
- unsigned long flags; /* TTY flags ASY_*/
+ unsigned long flags; /* User TTY flags ASYNC_ */
+ unsigned long iflags; /* Kernel internal flags ASY_ */
unsigned char console:1, /* port is a console */
low_latency:1; /* optional: tune for latency */
struct mutex mutex; /* Locking */
@@ -242,6 +243,19 @@ struct tty_port {
struct kref kref; /* Ref counter */
};
+/* tty_port::iflags bits -- use atomic bit ops */
+#define ASY_ON 0 /* device is initialized */
+#define ASY_SUSPENDED 1 /* device is suspended */
+#define ASY_ACTIVE 2 /* device is open */
+
+/*
+ * uart drivers: use the uart_port::status field and the UPSTAT_* defines
+ * for s/w-based flow control steering and carrier detection status
+ */
+#define ASY_CTS_FLOW 3 /* h/w flow control enabled */
+#define ASY_CHECK_CD 4 /* carrier detect enabled */
+
+
/*
* Where all of the state associated with a tty is kept while the tty
* is open. Since the termios state should be kept even if the tty
diff --git a/include/uapi/linux/tty_flags.h b/include/uapi/linux/tty_flags.h
index 072e41e..b004201 100644
--- a/include/uapi/linux/tty_flags.h
+++ b/include/uapi/linux/tty_flags.h
@@ -32,7 +32,12 @@
#define ASYNCB_MAGIC_MULTIPLIER 16 /* Use special CLK or divisor */
#define ASYNCB_LAST_USER 16
-/* Internal flags used only by kernel */
+/*
+ * Internal flags used only by kernel (read-only)
+ *
+ * WARNING: These flags are no longer used and have been superceded by the
+ * private ASY_* flags in the iflags field (and not userspace-visible)
+ */
#define ASYNCB_INITIALIZED 31 /* Serial port was initialized */
#define ASYNCB_SUSPENDED 30 /* Serial port is suspended */
#define ASYNCB_NORMAL_ACTIVE 29 /* Normal device is active */
@@ -44,6 +49,7 @@
#define ASYNCB_CONS_FLOW 23 /* flow control for console */
#define ASYNCB_FIRST_KERNEL 22
+/* Masks */
#define ASYNC_HUP_NOTIFY (1U << ASYNCB_HUP_NOTIFY)
#define ASYNC_SUSPENDED (1U << ASYNCB_SUSPENDED)
#define ASYNC_FOURPORT (1U << ASYNCB_FOURPORT)
@@ -72,6 +78,7 @@
#define ASYNC_SPD_WARP (ASYNC_SPD_HI|ASYNC_SPD_SHI)
#define ASYNC_SPD_MASK (ASYNC_SPD_HI|ASYNC_SPD_VHI|ASYNC_SPD_SHI)
+/* These flags are no longer used (and were always masked from userspace) */
#define ASYNC_INITIALIZED (1U << ASYNCB_INITIALIZED)
#define ASYNC_NORMAL_ACTIVE (1U << ASYNCB_NORMAL_ACTIVE)
#define ASYNC_BOOT_AUTOCONF (1U << ASYNCB_BOOT_AUTOCONF)
--
2.7.0
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