linux-kernel - Re: [PATCH 34/35] tty: make tty_get_byte

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Date:   Thu, 6 May 2021 09:19:32 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Joe Perches <joe@...ches.com>, gregkh@...uxfoundation.org
Cc:     linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 34/35] tty: make tty_get_byte_size available

On 06. 05. 21, 9:16, Joe Perches wrote:
> On Wed, 2021-05-05 at 11:19 +0200, Jiri Slaby wrote:
>> Many tty drivers contain code to compute bits count depending on termios
>> cflags. So extract this code from serial core to a separate tty helper
>> function called tty_get_byte_size.
> []
>> diff --git a/drivers/tty/tty_ioctl.c b/drivers/tty/tty_ioctl.c
> []
>> +/**
>> + *	tty_get_byte_size	-	get size of a byte
>> + *	@cflag: termios cflag value
>> + *	@account_flags: account for start and stop bits, second stop bit (if
>> + *			set), and parity (if set)
>> + *
>> + *	Get the size of a byte in bits depending on @cflag. Depending on
>> + *	@account_flags parameter, the result also accounts start and stop bits,
>> + *	the second stop bit, and parity bit.
>> + */
>> +unsigned char tty_get_byte_size(unsigned int cflag, bool account_flags)
>> +{
>> +	unsigned char bits = account_flags ? 2 : 0;
>> +
>> +	/* byte size and parity */
>> +	switch (cflag & CSIZE) {
>> +	case CS5:
>> +		bits += 5;
>> +		break;
>> +	case CS6:
>> +		bits += 6;
>> +		break;
>> +	case CS7:
>> +		bits += 7;
>> +		break;
>> +	case CS8:
>> +	default:
>> +		bits += 8;
>> +		break;
>> +	}
>> +
>> +	if (account_flags && (cflag & CSTOPB))
>> +		bits++;
>> +
>> +	if (account_flags && (cflag & PARENB))
>> +		bits++;
>> +
>> +	return bits;
>> +}
>> +EXPORT_SYMBOL_GPL(tty_get_byte_size);
> 
> Perhaps clearer not testing account_flags multiple times.

Right. Originally, I had account_start_stop, account_stop, and 
account_parity parameters. But they were either all false or all true. 
So I unified them to account_flags, but the code remained overly complex.

> unsigned char tty_get_byte_size(unsigned int cflag, bool account_flags)
> {
> 	unsigned char bits;
> 
> 	/* byte size and parity */
> 	switch (cflag & CSIZE) {
> 	case CS5:
> 		bits = 5;
> 		break;
> 	case CS6:
> 		bits = 6;
> 		break;
> 	case CS7:
> 		bits = 7;
> 		break;
> 	case CS8:
> 	default:
> 		bits = 8;
> 		break;
> 	}
> 
> 	if (account_flags) {
> 		bits += 2;	/* start/stop bits */
> 
> 		if (cflag & CSTOPB)
> 			bits++;
> 
> 		if (cflag & PARENB)
> 			bits++;
> 	}
> 
> 	return bits;
> }
> 
> 

thanks,
-- 
js
suse labs