8250_dw bug

8250_dw bug

[Nuno Gonçalves]

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

Re: 8250_dw bug

[Greg KH]

On Thu, Jan 11, 2018 at 01:47:31PM +0100, Nuno Gonçalves wrote:
> 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.

Why wouldn't you submit a "formal patch", that way we could apply it if
it is correct :)

also cc: the linux-serial mailing list when you do so, thanks.

greg k-h