76 lines
3.1 KiB
Diff
76 lines
3.1 KiB
Diff
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From: John Ogness <john.ogness@linutronix.de>
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Date: Tue, 20 Aug 2024 08:35:33 +0206
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Subject: [PATCH 07/54] printk: nbcon: Add detailed doc for write_atomic()
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Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.11/older/patches-6.11-rt7.tar.xz
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The write_atomic() callback has special requirements and is
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allowed to use special helper functions. Provide detailed
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documentation of the callback so that a developer has a
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chance of implementing it correctly.
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Signed-off-by: John Ogness <john.ogness@linutronix.de>
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Reviewed-by: Petr Mladek <pmladek@suse.com>
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Link: https://lore.kernel.org/r/20240820063001.36405-8-john.ogness@linutronix.de
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Signed-off-by: Petr Mladek <pmladek@suse.com>
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Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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---
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include/linux/console.h | 33 +++++++++++++++++++++++++++++----
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1 file changed, 29 insertions(+), 4 deletions(-)
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--- a/include/linux/console.h
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+++ b/include/linux/console.h
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@@ -303,7 +303,7 @@ struct nbcon_write_context {
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/**
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* struct console - The console descriptor structure
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* @name: The name of the console driver
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- * @write: Write callback to output messages (Optional)
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+ * @write: Legacy write callback to output messages (Optional)
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* @read: Read callback for console input (Optional)
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* @device: The underlying TTY device driver (Optional)
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* @unblank: Callback to unblank the console (Optional)
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@@ -320,7 +320,6 @@ struct nbcon_write_context {
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* @data: Driver private data
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* @node: hlist node for the console list
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*
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- * @write_atomic: Write callback for atomic context
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* @nbcon_state: State for nbcon consoles
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* @nbcon_seq: Sequence number of the next record for nbcon to print
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* @pbufs: Pointer to nbcon private buffer
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@@ -345,8 +344,34 @@ struct console {
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struct hlist_node node;
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/* nbcon console specific members */
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- void (*write_atomic)(struct console *con,
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- struct nbcon_write_context *wctxt);
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+
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+ /**
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+ * @write_atomic:
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+ *
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+ * NBCON callback to write out text in any context. (Optional)
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+ *
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+ * This callback is called with the console already acquired. However,
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+ * a higher priority context is allowed to take it over by default.
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+ *
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+ * The callback must call nbcon_enter_unsafe() and nbcon_exit_unsafe()
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+ * around any code where the takeover is not safe, for example, when
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+ * manipulating the serial port registers.
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+ *
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+ * nbcon_enter_unsafe() will fail if the context has lost the console
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+ * ownership in the meantime. In this case, the callback is no longer
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+ * allowed to go forward. It must back out immediately and carefully.
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+ * The buffer content is also no longer trusted since it no longer
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+ * belongs to the context.
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+ *
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+ * The callback should allow the takeover whenever it is safe. It
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+ * increases the chance to see messages when the system is in trouble.
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+ *
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+ * The callback can be called from any context (including NMI).
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+ * Therefore it must avoid usage of any locking and instead rely
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+ * on the console ownership for synchronization.
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+ */
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+ void (*write_atomic)(struct console *con, struct nbcon_write_context *wctxt);
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+
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atomic_t __private nbcon_state;
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atomic_long_t __private nbcon_seq;
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struct printk_buffers *pbufs;
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