linux-kernel - Re: [PATCH v2 6/7] serial: liteuart: separate RX loop from poll timer

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Date:   Thu, 10 Nov 2022 15:29:30 -0500
From:   "Gabriel L. Somlo" <gsomlo@...il.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-serial <linux-serial@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>, kgugala@...micro.com,
        mholenko@...micro.com, joel@....id.au,
        david.abdurachmanov@...il.com, florent@...oy-digital.fr,
        geert@...ux-m68k.org
Subject: Re: [PATCH v2 6/7] serial: liteuart: separate RX loop from poll timer

On Thu, Nov 10, 2022 at 01:01:47PM +0200, Ilpo Järvinen wrote:
> On Wed, 9 Nov 2022, Gabriel Somlo wrote:
> 
> > Move the character-receive (RX) loop to its own dedicated function,
> > and (for now) call that from the poll timer, liteuart_timer().
> > 
> > This is in preparation for adding IRQ support to the receive path.
> > 
> > Signed-off-by: Gabriel Somlo <gsomlo@...il.com>
> > ---
> >  drivers/tty/serial/liteuart.c | 25 +++++++++++++++----------
> >  1 file changed, 15 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/tty/serial/liteuart.c b/drivers/tty/serial/liteuart.c
> > index 047d5ad32e13..aa7052280197 100644
> > --- a/drivers/tty/serial/liteuart.c
> > +++ b/drivers/tty/serial/liteuart.c
> > @@ -67,29 +67,34 @@ static struct uart_driver liteuart_driver = {
> >  #endif
> >  };
> >  
> > -static void liteuart_timer(struct timer_list *t)
> > +static void liteuart_rx_chars(struct uart_port *port)
> >  {
> > -	struct liteuart_port *uart = from_timer(uart, t, timer);
> > -	struct uart_port *port = &uart->port;
> >  	unsigned char __iomem *membase = port->membase;
> > -	unsigned int flg = TTY_NORMAL;
> > -	int ch;
> > -	unsigned long status;
> > +	unsigned int status;
> > +	unsigned char ch;
> >  
> >  	while ((status = !litex_read8(membase + OFF_RXEMPTY)) == 1) {
> >  		ch = litex_read8(membase + OFF_RXTX);
> >  		port->icount.rx++;
> >  
> >  		/* necessary for RXEMPTY to refresh its value */
> > -		litex_write8(membase + OFF_EV_PENDING, EV_TX | EV_RX);
> > +		litex_write8(membase + OFF_EV_PENDING, EV_RX);
> >  
> >  		/* no overflow bits in status */
> >  		if (!(uart_handle_sysrq_char(port, ch)))
> > -			uart_insert_char(port, status, 0, ch, flg);
> > -
> > -		tty_flip_buffer_push(&port->state->port);
> > +			uart_insert_char(port, status, 0, ch, TTY_NORMAL);
> >  	}
> >  
> > +	tty_flip_buffer_push(&port->state->port);
> 
> This change is doing extra stuff besides moving rx to a dedicated 
> function.
> 
> I see no reason why those other changes couldn't be put into an entirely 
> separate patch. Also, please described those changes properly in the 
> commit message (answer the why? question).
 
You're right, calling `tty_flip_buffer_push()` as each character is
received is overkill, we only need to call it once per interrupt once
all available characters have been received.

I forgot I noticed (and fixed) that as part of the move -- I'll split
it out into its own separate patch (probably *before* moving all of rx
to a dedicated function).

Should show up in v3, once I also address all the other feedback I
received.

Thanks again for catching it!

Cheers,
-Gabriel

> -- 
>  i.
> 
> > +}
> > +
> > +static void liteuart_timer(struct timer_list *t)
> > +{
> > +	struct liteuart_port *uart = from_timer(uart, t, timer);
> > +	struct uart_port *port = &uart->port;
> > +
> > +	liteuart_rx_chars(port);
> > +
> >  	mod_timer(&uart->timer, jiffies + uart_poll_timeout(port));
> >  }
> >  
> > 
> 
>