RE: [PATCH v2 1/2] serial: sh-sci: Avoid divide-by-zero fault

[Biju Das]

Hi Geert,

Thanks for the feedback.

> -----Original Message-----
> From: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> Sent: 13 April 2026 11:25
> Subject: Re: [PATCH v2 1/2] serial: sh-sci: Avoid divide-by-zero fault
> 
> Hi Biju,
> 
> On Wed, 8 Apr 2026 at 19:25, Biju Das <biju.das.jz@xxxxxxxxxxxxxx> wrote:
> > > From: Hugo Villeneuve <hugo@xxxxxxxxxxx> On Wed, 8 Apr 2026 16:35:44
> > > +0000 Biju Das <biju.das.jz@xxxxxxxxxxxxxx> wrote:
> > > > > From: Hugo Villeneuve <hugo@xxxxxxxxxxx> Biju
> > > > > <biju.das.au@xxxxxxxxx> wrote:
> > > > > > From: Biju Das <biju.das.jz@xxxxxxxxxxxxxx>
> > > > > >
> > > > > > uart_update_timeout() computes a timeout value by dividing by
> > > > > > the baud rate. If baud is zero — which can occur when the
> > > > > > hardware returns an unsupported or invalid rate — this results in a divide-by-zero fault.
> > > > >
> > > > > baud is returned by uart_get_baud_rate(), so this is not returned by the hardware?
> > > >
> > > > You are tight, Will update commit description.
> > >
> > > How can uart_get_baud_rate() return a zero value? If I am not
> > > mistaken even for the B0 case, it will return 9600?
> >
> > As per the comment and code, this API can return 0.
> >
> > * If the new baud rate is invalid, try the @old termios setting. If
> > it's still
> > * invalid, we try 9600 baud. If that is also invalid 0 is returned.
> >
> > In drives/tty currently only 1 driver is checking the return value and
> > it calls panic
> >
> > https://elixir.bootlin.com/linux/v7.0-rc7/source/drivers/tty/serial/ap
> > buart.c#L214
> >
> >
> > I believe we should call panic, if baud =0, instead of proceeding.
> >
> > Geert, any thoughts??
> 
> IIRC, baud == 0 can (only?) happen when using earlyprintk on a non-DT system, where the serial console
> should just keep on using the settings programmed by the firmware.  So any config register writes
> should be skipped.
> 
> On DT systems, even earlycon uses the bitrate from chosen/stdout-path.

In that case, I will drop the zero baud check.

I will add a new patch with below change which is algebraically equivalent
but eliminates the intermediate division, making a zero divisor impossible
for any valid baud rate.

-	s->rx_frame = (10000 * bits) / (baud / 100);
+	s->rx_frame = (10000 * bits) * 100 / baud;

Cheers,
Biju