linux-kernel - Re: [PATCH v1 07/31] serial: 8250_mxupci: add GDL-based Rx routine for 8250

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Message-ID: <CAHp75Vex-YzE-0PydYcSZGd24hkmbBanVHRTSsWQ_X-bc8kW9Q@mail.gmail.com>
Date: Sun, 30 Nov 2025 19:29:47 +0200
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Crescent Hsieh <crescentcy.hsieh@...a.com>
Cc: gregkh@...uxfoundation.org, jirislaby@...nel.org, 
	ilpo.jarvinen@...ux.intel.com, linux-kernel@...r.kernel.org, 
	linux-serial@...r.kernel.org
Subject: Re: [PATCH v1 07/31] serial: 8250_mxupci: add GDL-based Rx routine
 for 8250_mxupci

On Sun, Nov 30, 2025 at 12:43 PM Crescent Hsieh
<crescentcy.hsieh@...a.com> wrote:
>
> Introduce a custom Rx routine using the GDL (Good Data Length) register to
> read multiple bytes from the Rx FIFO on Moxa UPCI serial boards.
>
> When no LSR error bits are present, the handler reads GDL bytes from
> UART_RX and inserts them into the TTY flip buffer. If error bits are
> detected, it falls back to the default serial8250_rx_chars() handler.

...

> +static void mxupci8250_rx_chars(struct uart_8250_port *up)
> +{
> +       struct uart_port *port = &up->port;
> +       struct tty_port *tport = &port->state->port;
> +       int recv_room, gdl, i;
> +       u8 buf[MOXA_UART_FIFO_SIZE];
> +
> +       recv_room = tty_buffer_request_room(tport, port->fifosize);

> +

Stray blank line addition.

> +       if (recv_room) {

Can't it be

  if (!recv_room)
    return;

?

> +               gdl = serial_in(up, MOXA_UART_GDL);

> +

Stray blank line addition.

> +               if (gdl > recv_room)
> +                       gdl = recv_room;
> +
> +               for (i = 0; i < gdl; ++i)
> +                       buf[i] = serial_in(up, UART_RX);
> +
> +               port->icount.rx += gdl;
> +               tty_insert_flip_string(tport, buf, gdl);
> +               tty_flip_buffer_push(tport);
> +       }
> +}

...

> -       if (lsr & (UART_LSR_DR | UART_LSR_BI) && !skip_rx)
> -               lsr = serial8250_rx_chars(up, lsr);
> -
> +       if (lsr & (UART_LSR_DR | UART_LSR_BI) && !skip_rx) {
> +               if (lsr & UART_LSR_BRK_ERROR_BITS)
> +                       lsr = serial8250_rx_chars(up, lsr);
> +               else
> +                       mxupci8250_rx_chars(up);
> +       }

Oh, can we reduce ping-pong a bit (the modification of the lines just
being added earlier in the same patch series)?

I think you can create a helper to wrap 8250_rx_chars() with split
version of the almost unreadable conditionals, this will also remove
the skip_rx variable

static ... _rx_chars()
{
  /* split version of skip_rx monstrous if */
  if ...
    return ...;
  if ...
   return ...;

  if ...
   return ...;

  return 8250_rx_chars(...);
}

Then in this patch only a couple of lines will be added to the end of
the function.

  if (...)
   return ...

-- 
With Best Regards,
Andy Shevchenko