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From:   Dawei Li <dawei.li@shingroup.cn>
To:     gregkh@linuxfoundation.org, ilpo.jarvinen@linux.intel.com,
        andriy.shevchenko@linux.intel.com
Cc:     jirislaby@kernel.org, jszhang@kernel.org,
        linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org,
        set_pte_at@outlook.com, Dawei Li <dawei.li@shingroup.cn>,
        stable@kernel.org
Subject: [PATCH] serial: dw8250: Make DLF feature independent of ADDITIONAL_FEATURE
Date:   Mon,  4 Dec 2023 21:08:20 +0800
Message-Id: <20231204130820.2823688-1-dawei.li@shingroup.cn>
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Feedback-ID: bizesmtp:shingroup.cn:qybglogicsvrgz:qybglogicsvrgz5a-1
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DW apb uart databook defines couples of configuration parameters of
dw8250 IP, among which there are 2 of them:
- ADDTIONAL_FEATURE
" Configure the peripheral to have the option to include FIFO status
  registers, shadow registers and encoded parameter register. Also
  configures the peripheral to have UART component version and the
  peripheral ID registers. "

- FRACTIONAL_BAUD_DIVISOR_EN
" Configures the peripheral to have Fractional Baud Rate Divisor.
  .... "

These two parameters are completely irrelevant, and supposed to be
independent of each other. However, in current dw8250 driver
implementation, they are hooked together.

The bug was hit when we are bringing up dw8250 IP on our hardware
platform, in which parameters are configured in such combination:
- ADDTIONAL_FEATURE disabled;
- FRACTIONAL_BAUD_DIVISOR_EN enabled;

Fixes: 701c5e73b296 ("serial: 8250_dw: add fractional divisor support")
Cc: stable@kernel.org
Signed-off-by: Dawei Li <dawei.li@shingroup.cn>
---
 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;
-- 
2.27.0