Re: [PATCH v7 tty-next 3/4] serial: 8250_pci1xxxx: Add RS485 support to quad-uart driver

From: "Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>
To: Kumaravel Thiagarajan <kumaravel.thiagarajan@microchip.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	linux-serial <linux-serial@vger.kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jirislaby@kernel.org>,
	macro@orcam.me.uk,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	cang1@live.co.uk, colin.i.king@gmail.com,
	phil.edworthy@renesas.com, biju.das.jz@bp.renesas.com,
	geert+renesas@glider.be, Lukas Wunner <lukas@wunner.de>,
	u.kleine-koenig@pengutronix.de, wander@redhat.com,
	etremblay@distech-controls.com, jk@ozlabs.org,
	Tharun Kumar P <tharunkumar.pasumarthi@microchip.com>
Subject: Re: [PATCH v7 tty-next 3/4] serial: 8250_pci1xxxx: Add RS485 support to quad-uart driver
Date: Wed, 7 Dec 2022 13:53:17 +0200 (EET)	[thread overview]
Message-ID: <784d1b68-98b8-324-ad34-7ef13352855@linux.intel.com> (raw)
In-Reply-To: <20221207235305.695541-4-kumaravel.thiagarajan@microchip.com>

On Thu, 8 Dec 2022, Kumaravel Thiagarajan wrote:

> pci1xxxx uart supports RS485 mode of operation in the hardware with
> auto-direction control with configurable delay for releasing RTS after
> the transmission. This patch adds support for the RS485 mode.
> 
> Co-developed-by: Tharun Kumar P <tharunkumar.pasumarthi@microchip.com>
> Signed-off-by: Tharun Kumar P <tharunkumar.pasumarthi@microchip.com>
> Signed-off-by: Kumaravel Thiagarajan <kumaravel.thiagarajan@microchip.com>
> ---
> Changes in v7:
> - No Change
> 
> Changes in v6:
> - Modified datatype of delay_in_baud_periods to u64 to avoid overflows
> 
> Changes in v5:
> - Removed unnecessary assignments
> - Corrected styling issues in comments
> 
> Changes in v4:
> - No Change
> 
> Changes in v3:
> - Remove flags sanitization in driver which is taken care in core
> 
> Changes in v2:
> - move pci1xxxx_rs485_config to a separate patch with
>   pci1xxxx_rs485_supported.
> ---
>  drivers/tty/serial/8250/8250_pci1xxxx.c | 49 +++++++++++++++++++++++++
>  1 file changed, 49 insertions(+)
> 
> diff --git a/drivers/tty/serial/8250/8250_pci1xxxx.c b/drivers/tty/serial/8250/8250_pci1xxxx.c
> index be554e2d884b..9f0da264314a 100644
> --- a/drivers/tty/serial/8250/8250_pci1xxxx.c
> +++ b/drivers/tty/serial/8250/8250_pci1xxxx.c
> @@ -145,6 +145,53 @@ static void pci1xxxx_set_divisor(struct uart_port *port, unsigned int baud,
>  	       port->membase + UART_BAUD_CLK_DIVISOR_REG);
>  }
>  
> +static int pci1xxxx_rs485_config(struct uart_port *port,
> +				 struct ktermios *termios,
> +				 struct serial_rs485 *rs485)
> +{
> +	u32 clock_div = readl(port->membase + UART_BAUD_CLK_DIVISOR_REG);
> +	u64 delay_in_baud_periods;
> +	u32 baud_period_in_ns;
> +	u32 data = 0;
> +
> +	/*
> +	 * pci1xxxx's uart hardware supports only RTS delay after
> +	 * Tx and in units of bit times to a maximum of 15
> +	 */
> +	if (rs485->flags & SER_RS485_ENABLED) {
> +		data = ADCL_CFG_EN | ADCL_CFG_PIN_SEL;
> +
> +		if (!(rs485->flags & SER_RS485_RTS_ON_SEND))
> +			data |= ADCL_CFG_POL_SEL;
> +
> +		if (rs485->delay_rts_after_send) {
> +			baud_period_in_ns =
> +				FIELD_GET(BAUD_CLOCK_DIV_INT_MSK, clock_div) *
> +				UART_BIT_SAMPLE_CNT;
> +			delay_in_baud_periods =
> +				rs485->delay_rts_after_send * NSEC_PER_MSEC /
> +				baud_period_in_ns;
> +			delay_in_baud_periods =
> +				min_t(u64, delay_in_baud_periods,
> +				      FIELD_MAX(ADCL_CFG_RTS_DELAY_MASK));
> +			data |= FIELD_PREP(ADCL_CFG_RTS_DELAY_MASK,
> +					   delay_in_baud_periods);
> +			rs485->delay_rts_after_send =
> +				baud_period_in_ns * delay_in_baud_periods /
> +				NSEC_PER_MSEC;

div_u64() or cast+comment if you can prove you never need the high-word at 
this point.

But why you even need u64 because delay_rts_after_send is limited to 
RS485_MAX_RTS_DELAY (=100) by serial core? 100 * NSEC_PER_MSEC doesn't 
overflow u32.

-- 
 i.