linux-kernel - 8250

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Date:   Thu, 11 Jan 2018 13:47:31 +0100
From:   Nuno Gonçalves <nunojpg@...il.com>
To:     ed.blake@...drel.com, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org
Subject: 8250_dw bug

Dear Ed and Greg,

There is a small bug on de9e33bdfa22e607a88494ff21e9196d00bf4550, at
least on 32bit devices.

Line 274 if (rate >= i * min_rate && rate <= i * max_rate)

This will overflow when min_rate/max_rate is large and can not be
achieved in the hardware.

Eg.

stty -F /dev/ttyS2 raw 3500000 (not achievable on my board).

target_rate=56000000 (for 3500000baud)
min_rate=55125000
max_rate=56875000
rate=24000000 (clk_round_rate for my board)

Since my board can only do 1500000baud, this loop will keep
incrementing i until i=77*56875000 will overflow, and there are
unexpected results.

I suggest this patch:

--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -267,7 +267,13 @@ static void dw8250_set_termios(struct uart_port
*p, struct ktermios *termios,

        for (i = 1; i <= UART_DIV_MAX; i++) {
                rate = clk_round_rate(d->clk, i * target_rate);
-               if (rate >= i * min_rate && rate <= i * max_rate)
+
+               if (rate < i * min_rate) {
+                       i = UART_DIV_MAX;
+                       break;
+               }
+
+               if (rate <= i * max_rate)
                        break;
        }
        if (i <= UART_DIV_MAX) {

Let me know if you want me to submit the formal patch.

Thanks,
Nuno