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

[Hugo Villeneuve]

Hi Biju,

On Thu, 9 Apr 2026 07:40:02 +0000
Biju Das <biju.das.jz@xxxxxxxxxxxxxx> wrote:

> 
> 
> > -----Original Message-----
> > From: Biju Das
> > Sent: 08 April 2026 20:02
> > To: Hugo Villeneuve <hugo@xxxxxxxxxxx>
> > Cc: biju.das.au <biju.das.au@xxxxxxxxx>; Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>; Jiri Slaby
> > <jirislaby@xxxxxxxxxx>; Geert Uytterhoeven <geert+renesas@xxxxxxxxx>; Thierry Bultel
> > <thierry.bultel.yh@xxxxxxxxxxxxxx>; wsa+renesas <wsa+renesas@xxxxxxxxxxxxxxxxxxxx>; Prabhakar Mahadev
> > Lad <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; linux-
> > serial@xxxxxxxxxxxxxxx; linux-renesas-soc@xxxxxxxxxxxxxxx
> > Subject: RE: [PATCH v2 1/2] serial: sh-sci: Avoid divide-by-zero fault
> > 
> > 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.
> 
> May be setting a buadrate 115200 is safe in this cas like earlyprintk??
> I will send next version setting buad = 115200, if uart_get_baud_rate() returns 0.

Is it logical to proceed with configuration if it returned zero?

Also we still pass 0 as a minimum value in uart_get_baud_rate(), so
that a baud rate of 75, for example, would be valid, but would trigger a
fault later (division by zero). Wouldn't it be a good idea to also set a
proper minimum baud rate?

-- 
Hugo Villeneuve