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

[Biju Das]

Hi Hugo,

> -----Original Message-----
> From: Hugo Villeneuve <hugo@xxxxxxxxxxx>
> Sent: 08 April 2026 19:15
> Subject: Re: [PATCH v2 1/2] serial: sh-sci: Avoid divide-by-zero fault
> 
> Hi Biju,
> 
> On Wed, 8 Apr 2026 17:25:19 +0000
> Biju Das <biju.das.jz@xxxxxxxxxxxxxx> wrote:
> 
> > Hi Hugo,
> >
> > > -----Original Message-----
> > > From: Hugo Villeneuve <hugo@xxxxxxxxxxx>
> > > Sent: 08 April 2026 17:52
> > > Subject: Re: [PATCH v2 1/2] serial: sh-sci: Avoid divide-by-zero
> > > fault
> > >
> > > Hi Biju,
> > >
> > > On Wed, 8 Apr 2026 16:35:44 +0000
> > > Biju Das <biju.das.jz@xxxxxxxxxxxxxx> wrote:
> > >
> > > > Hi Hugo,
> > > >
> > > > Thanks for the feedback.
> > > >
> > > > > -----Original Message-----
> > > > > From: Hugo Villeneuve <hugo@xxxxxxxxxxx>
> > > > > Sent: 08 April 2026 17:31
> > > > > Subject: Re: [PATCH v2 1/2] serial: sh-sci: Avoid divide-by-zero
> > > > > fault
> > > > >
> > > > > Hi Biju,
> > > > >
> > > > > On Wed,  8 Apr 2026 15:20:58 +0100 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
> 
> Hmmm, more than 1:

> 
> icom.c:
>     if (!baud)
>          baud = 9600;    /* B0 transition handled in rs_set_termios */

A zero return from uart_get_baud_rate() is a normal, recoverable condition
(unsupported rate requested by userspace) and must not crash the kernel.

Or drop the check like other tty drivers, as SCIF/RSCI IP support 9600 baud rate.

Cheers,
Biju