[PATCH printk v5 06/30] printk: nbcon: Add callbacks to synchronize with driver

[John Ogness]

Console drivers typically must deal with access to the hardware
via user input/output (such as an interactive login shell) and
output of kernel messages via printk() calls. To provide the
necessary synchronization, usually some driver-specific locking
mechanism is used (for example, the port spinlock for uart
serial consoles).

Until now, usage of this driver-specific locking has been hidden
from the printk-subsystem and implemented within the various
console callbacks. However, for nbcon consoles, it is necessary
that the printk-subsystem uses the driver-specific locking so
that nbcon console ownership can be acquired _after_ the
driver-specific locking has succeeded. This allows for lock
contention to exist on the more context-friendly driver-specific
locking rather than nbcon console ownership (for non-emergency
and non-panic cases).

Require nbcon consoles to implement two new callbacks
(device_lock(), device_unlock()) that will use whatever
synchronization mechanism the driver is using for itself.

Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
---
 include/linux/console.h | 43 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/include/linux/console.h b/include/linux/console.h
index 3291cc340f1a..33a029d976c3 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -354,6 +354,49 @@ struct console {
 	 */
 	void (*write_atomic)(struct console *con, struct nbcon_write_context *wctxt);
 
+	/**
+	 * @device_lock:
+	 *
+	 * NBCON callback to begin synchronization with driver code.
+	 *
+	 * Console drivers typically must deal with access to the hardware
+	 * via user input/output (such as an interactive login shell) and
+	 * output of kernel messages via printk() calls. This callback is
+	 * called by the printk-subsystem whenever it needs to synchronize
+	 * with hardware access by the driver. It should be implemented to
+	 * use whatever synchronization mechanism the driver is using for
+	 * itself (for example, the port lock for uart serial consoles).
+	 *
+	 * The callback is always called from task context. It may use any
+	 * synchronization method required by the driver.
+	 *
+	 * IMPORTANT: The callback MUST disable migration. The console driver
+	 *	may be using a synchronization mechanism that already takes
+	 *	care of this (such as spinlocks). Otherwise this function must
+	 *	explicitly call migrate_disable().
+	 *
+	 * The flags argument is provided as a convenience to the driver. It
+	 * will be passed again to device_unlock(). It can be ignored if the
+	 * driver does not need it.
+	 */
+	void (*device_lock)(struct console *con, unsigned long *flags);
+
+	/**
+	 * @device_unlock:
+	 *
+	 * NBCON callback to finish synchronization with driver code.
+	 *
+	 * It is the counterpart to device_lock().
+	 *
+	 * This callback is always called from task context. It must
+	 * appropriately re-enable migration (depending on how device_lock()
+	 * disabled migration).
+	 *
+	 * The flags argument is the value of the same variable that was
+	 * passed to device_lock().
+	 */
+	void (*device_unlock)(struct console *con, unsigned long flags);
+
 	atomic_t		__private nbcon_state;
 	atomic_long_t		__private nbcon_seq;
 	struct printk_buffers	*pbufs;
-- 
2.39.2