linux-kernel - [PATCH v3 4/6] serial: 8250: Adjust misleading LSR related comment

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Date:   Tue,  7 Jun 2022 11:29:32 +0300
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     linux-serial@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, linux-kernel@...r.kernel.org
Cc:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Subject: [PATCH v3 4/6] serial: 8250: Adjust misleading LSR related comment

serial8250_rx_chars() has max_count based character limit. If it
triggers, the function returns old LSR value (and it has never returned
only flags which were not handled). Adjust the comment to match
behavior and warn about which flags can be depended on.

While I'd have moved LSR read before LSR read and used serial_lsr_in()
also here but I came across this old discussion about the topic:
  https://www.spinics.net/lists/linux-serial/msg20555.html
...so I left it as it is (it works as long as the callers only use
a subset of the LSR flags which holds true today).

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
---
 drivers/tty/serial/8250/8250_port.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index a0ea048eb2ad..686891f1b2ca 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1782,9 +1782,12 @@ void serial8250_read_char(struct uart_8250_port *up, unsigned char lsr)
 EXPORT_SYMBOL_GPL(serial8250_read_char);
 
 /*
- * serial8250_rx_chars: processes according to the passed in LSR
- * value, and returns the remaining LSR bits not handled
- * by this Rx routine.
+ * serial8250_rx_chars: Read characters. The first LSR value must be passed
+ * in.
+ *
+ * Returns LSR bits. The caller should rely only non-rx related LSR bits
+ * (such as THRE) because the LSR value might come from an already consumed
+ * character.
  */
 unsigned char serial8250_rx_chars(struct uart_8250_port *up, unsigned char lsr)
 {
-- 
2.30.2