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

John Ogness
2024-05-02T21:38:48Z

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@linutronix.de>
---
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