Re: [PATCH 7/8] serial: imx: use readl() to optimize FIFO reading loop

[Sergey Organov <sorganov@gmail.com>]

Sherry Sun <sherry.sun@nxp.com> writes:

>> -----Original Message-----
>> From: Sergey Organov <sorganov@gmail.com>
>> Sent: 2023年1月14日 2:44
>> To: linux-serial@vger.kernel.org
>> Cc: Fabio Estevam <festevam@gmail.com>; Greg Kroah-Hartman
>> <gregkh@linuxfoundation.org>; Jiri Slaby <jirislaby@kernel.org>; Richard
>> Genoud <richard.genoud@gmail.com>; Sascha Hauer
>> <s.hauer@pengutronix.de>; Shawn Guo <shawnguo@kernel.org>;
>> tharvey@gateworks.com; Tomasz Moń <tomasz.mon@camlingroup.com>;
>> linux-arm-kernel@lists.infradead.org; dl-linux-imx <linux-imx@nxp.com>;
>> Pengutronix Kernel Team <kernel@pengutronix.de>; Sergey Organov
>> <sorganov@gmail.com>
>> Subject: [PATCH 7/8] serial: imx: use readl() to optimize FIFO reading loop
>> 
>> Use readl() instead of heavier imx_uart_readl() in the Rx ISR, as we know we
>> read registers that must not be cached.
>> 
>> Signed-off-by: Sergey Organov <sorganov@gmail.com>
>> ---
>>  drivers/tty/serial/imx.c | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>> 
>> diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c index
>> be00362b8b67..f4236e8995fa 100644
>> --- a/drivers/tty/serial/imx.c
>> +++ b/drivers/tty/serial/imx.c
>> @@ -890,14 +890,15 @@ static irqreturn_t __imx_uart_rxint(int irq, void
>> *dev_id)
>>  	struct imx_port *sport = dev_id;
>>  	unsigned int rx, flg;
>>  	struct tty_port *port = &sport->port.state->port;
>> +	typeof(sport->port.membase) membase = sport->port.membase;
>>  	u32 usr2;
>> 
>>  	/* If we received something, check for 0xff flood */
>> -	usr2 = imx_uart_readl(sport, USR2);
>> +	usr2 = readl(membase + USR2);
>>  	if (usr2 & USR2_RDR)
>>  		imx_uart_check_flood(sport, usr2);
>> 
>> -	while ((rx = imx_uart_readl(sport, URXD0)) & URXD_CHARRDY) {
>> +	while ((rx = readl(membase + URXD0)) & URXD_CHARRDY) {
>
> Actually imx_uart_readl() only set shadow registers for UCRx and UFCR,
> for the USR2 and URXD0 that you used here, they will not be cached.

Sure, and that's why we here don't need to call imx_uart_readl(), that
only needlessly checks for shadowing, thus producing pure overhead.

Best Regards,
Sergey