linux-kernel - Re: [PATCH 3/3] serial: 8250_rt288x: Remove unnecessary UART_REG

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Message-ID: <9ad399f9-7d27-b541-2379-355f3a8258ae@linaro.org>
Date:   Fri, 30 Dec 2022 14:14:29 +0100
From:   Philippe Mathieu-Daudé <philmd@...aro.org>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-serial@...r.kernel.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     inux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] serial: 8250_rt288x: Remove unnecessary
 UART_REG_UNMAPPED

Hi Ilpo,

On 30/12/22 12:46, Ilpo Järvinen wrote:
> As unmapped registers are at the tail of the array, the ARRAY_SIZE()
> condition will catch them just fine. No need to define special
> value for them.

True but fragile example...

> Also, let the compiler to calculate the size of the array instead of
> providing it manually.
> 
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> ---
>   drivers/tty/serial/8250/8250_rt288x.c | 16 ++++------------
>   1 file changed, 4 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_rt288x.c b/drivers/tty/serial/8250/8250_rt288x.c
> index 3015afb99722..da8be9a802c1 100644
> --- a/drivers/tty/serial/8250/8250_rt288x.c
> +++ b/drivers/tty/serial/8250/8250_rt288x.c
> @@ -14,10 +14,8 @@
>   
>   #define RT288X_DL	0x28
>   
> -#define UART_REG_UNMAPPED	-1


> -static const s8 au_io_out_map[8] = {
> +static const s8 au_io_out_map[] = {
>   	[UART_TX]	= 1,
>   	[UART_IER]	= 2,
>   	[UART_FCR]	= 4,
>   	[UART_LCR]	= 5,
>   	[UART_MCR]	= 6,
> -	[UART_LSR]	= UART_REG_UNMAPPED,
> -	[UART_MSR]	= UART_REG_UNMAPPED,
> -	[UART_SCR]	= UART_REG_UNMAPPED,

If someone were to re-add an unlikely single

         [UART_SCR] = 42,

The array will also contain these hidden entries:

         [UART_LSR] = 0,
         [UART_MSR] = 0,

And these 2 registers end mapped.

>   };

Trying to 'optimize' array size when the array is index-initialized
can be bug-prone.

>   static unsigned int au_serial_in(struct uart_port *p, int offset)
> @@ -44,8 +38,7 @@ static unsigned int au_serial_in(struct uart_port *p, int offset)
>   	if (offset >= ARRAY_SIZE(au_io_in_map))
>   		return UINT_MAX;
>   	offset = au_io_in_map[offset];
> -	if (offset == UART_REG_UNMAPPED)
> -		return UINT_MAX;
> +
>   	return __raw_readl(p->membase + (offset << p->regshift));
>   }
Regards,

Phil.