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printk: nbcon: Add detailed doc for write_atomic()

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BugLink: https://bugs.launchpad.net/bugs/2060704

The write_atomic() callback has special requirements and is
allowed to use special helper functions. Provide detailed
documentation of the callback so that a developer has a
chance of implementing it correctly.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Kevin Becker <kevin.becker@canonical.com>
This commit is contained in:
John Ogness
2024-02-07 18:38:14 +00:00
committed by Kevin Becker
parent 5da0d44e43
commit d4aaab3e41
1 changed files with 27 additions and 4 deletions
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include/linux/console.h
+27 -4
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@@ -285,7 +285,7 @@ struct nbcon_write_context {
/**
* struct console - The console descriptor structure
* @name: The name of the console driver
* @write: Write callback to output messages (Optional)
* @write: Legacy write callback to output messages (Optional)
* @read: Read callback for console input (Optional)
* @device: The underlying TTY device driver (Optional)
* @unblank: Callback to unblank the console (Optional)
@@ -302,7 +302,6 @@ struct nbcon_write_context {
* @data: Driver private data
* @node: hlist node for the console list
*
* @write_atomic: Write callback for atomic context
* @nbcon_state: State for nbcon consoles
* @nbcon_seq: Sequence number of the next record for nbcon to print
* @pbufs: Pointer to nbcon private buffer
@@ -327,8 +326,32 @@ struct console {
struct hlist_node node;
/* nbcon console specific members */
void (*write_atomic)(struct console *con,
struct nbcon_write_context *wctxt);
/**
* @write_atomic:
*
* NBCON callback to write out text in any context.
*
* This callback is called with the console already acquired. The
* callback can use nbcon_can_proceed() at any time to verify that
* it is still the owner of the console. In the case that it has
* lost ownership, it is no longer allowed to go forward. In this
* case it must back out immediately and carefully. The buffer
* content is also no longer trusted since it no longer belongs to
* the context.
*
* If the callback needs to perform actions where ownership is not
* allowed to be taken over, nbcon_enter_unsafe() and
* nbcon_exit_unsafe() can be used to mark such sections. These
* functions are also points of possible ownership transfer. If
* either function returns false, ownership has been lost.
*
* This callback can be called from any context (including NMI).
* Therefore it must avoid usage of any locking and instead rely
* on the console ownership for synchronization.
*/
void (*write_atomic)(struct console *con, struct nbcon_write_context *wctxt);
atomic_t __private nbcon_state;
atomic_long_t __private nbcon_seq;
struct printk_buffers *pbufs;