linux-kernel - [PATCH] serial: 8250_dw: Decouple DLF register check from UCV

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Message-ID: <20231126160420.2442330-1-vamshigajjela@google.com>
Date:   Sun, 26 Nov 2023 21:34:20 +0530
From:   Vamshi Gajjela <vamshigajjela@...gle.com>
To:     Jiri Slaby <jirislaby@...nel.org>, ilpo.jarvinen@...ux.intel.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        manugautam@...gle.com, Subhash Jadavani <sjadavani@...gle.com>,
        Vamshi Gajjela <vamshigajjela@...gle.com>
Subject: [PATCH] serial: 8250_dw: Decouple DLF register check from UCV

Designware UART has an optional feature to enable Fractional Baud Rate
Divisor (DLF) through the FRACTIONAL_BAUD_DIVISOR_EN configuration
parameter, and it is not dependent on ADDITIONAL_FEATURES.

dw8250_setup_port() checks DLF to determine dlf_size only when UART
Component Version (UCV) is non-zero. As mentioned above DLF and UCV are
independent features. Move the logic corresponding to DLF size
calculation ahead of the UCV check to prevent early return. Otherwise,
dlf_size will be zero and driver will not be able to use the
controller's fractional baud rate divisor (DLF) feature.

Signed-off-by: Vamshi Gajjela <vamshigajjela@...gle.com>
---
 drivers/tty/serial/8250/8250_dwlib.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dwlib.c b/drivers/tty/serial/8250/8250_dwlib.c
index 84843e204a5e..136ad093c5b6 100644
--- a/drivers/tty/serial/8250/8250_dwlib.c
+++ b/drivers/tty/serial/8250/8250_dwlib.c
@@ -259,17 +259,6 @@ void dw8250_setup_port(struct uart_port *p)
 	}
 	up->capabilities |= UART_CAP_NOTEMT;
 
-	/*
-	 * 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);
-
 	/* Preserve value written by firmware or bootloader  */
 	old_dlf = dw8250_readl_ext(p, DW_UART_DLF);
 	dw8250_writel_ext(p, DW_UART_DLF, ~0U);
@@ -282,6 +271,17 @@ 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);
+
 	reg = dw8250_readl_ext(p, DW_UART_CPR);
 	if (!reg) {
 		reg = data->pdata->cpr_val;
-- 
2.43.0.rc1.413.gea7ed67945-goog