linux-kernel - [PATCH] serial: 8250_dw: Do not bailout on UCV read returning zero

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Message-ID: <20231231182951.877805-1-vamshigajjela@google.com>
Date: Sun, 31 Dec 2023 23:59:51 +0530
From: Vamshi Gajjela <vamshigajjela@...gle.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby <jirislaby@...nel.org>, 
	ilpo.jarvinen@...ux.intel.com, 
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org, 
	manugautam@...gle.com, Vamshi Gajjela <vamshigajjela@...gle.com>
Subject: [PATCH] serial: 8250_dw: Do not bailout on UCV read returning zero

Designware UART has optional feature FIFO_MODE to implement FIFO.
Encoding FIFO capabilities through Component Parameter Register CPR is
optional and it can be enabled using parameter UART_ADD_ENCODED_PARAMS.

Driver can exercise fifo capabilities by decoding CPR if implemented
or from cpr_val provided from the dw8250_platform_data otherwise.

dw8250_setup_port() checks for CPR or cpr_val to determine FIFO size
only when Component Version (UCV) is non-zero. Bailing out early on UCV
read returning zero will leave fifosize as zero and !UART_CAP_FIFO,
hence prevent early return and continue to process CPR or cpr_val for
the driver to utilize FIFO.

Non-zero UCV implies ADDITIONAL_FEATURES=1, preventing early return
will not be an overhead here.

Signed-off-by: Vamshi Gajjela <vamshigajjela@...gle.com>
---
 drivers/tty/serial/8250/8250_dwlib.c | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dwlib.c b/drivers/tty/serial/8250/8250_dwlib.c
index 136ad093c5b6..3e33ddf7bc80 100644
--- a/drivers/tty/serial/8250/8250_dwlib.c
+++ b/drivers/tty/serial/8250/8250_dwlib.c
@@ -271,16 +271,10 @@ void dw8250_setup_port(struct uart_port *p)
 		p->set_divisor = dw8250_set_divisor;
 	}

-	/*
-	 * If the Component Version Register returns zero, we know that
-	 * ADDITIONAL_FEATURES are not enabled. No need to go any further.
-	 */
 	reg = dw8250_readl_ext(p, DW_UART_UCV);
-	if (!reg)
-		return;
-
-	dev_dbg(p->dev, "Designware UART version %c.%c%c\n",
-		(reg >> 24) & 0xff, (reg >> 16) & 0xff, (reg >> 8) & 0xff);
+	if (reg)
+		dev_dbg(p->dev, "Designware UART version %c.%c%c\n",
+			(reg >> 24) & 0xff, (reg >> 16) & 0xff, (reg >> 8) & 0xff);

 	reg = dw8250_readl_ext(p, DW_UART_CPR);
 	if (!reg) {
-- 
2.43.0.472.g3155946c3a-goog