[PATCH 4/4] lib: utils/serial: Ensure baudrate is non-zero before using

[Andrew Jones]

On Wed, Jul 20, 2022 at 11:12:18PM +0800, Xiang W wrote:
> 在 2022-07-18星期一的 19:20 +0200，Andrew Jones写道：
> > RISC-V doesn't generate exceptions on divide-by-zero, but the result,
> > all bits set, is not likely what people expect either. In all cases
> > where we divide by baudrate there's a chance it's zero (when the DT
> > it came from is "bad"). To avoid difficult to debug situations, leave
> > baudrate dependent registers alone when baudrate is zero, as, also in
> > all cases, it appears we can skip initialization of those registers
> > and still [hopefully] have a functioning UART.
> > 
> > Signed-off-by: Andrew Jones <ajones at ventanamicro.com>
> If the parameter is wrong we should skip initialization and should not
> execute sbi_console_set_device.

Hi Xiang,

While there should be some DT validation somewhere that points out bad
parameters, I don't think opensbi is the best place to do it, especially
for UART configuration. While we certainly can't progress in all cases
we detect bad input, in the case of a zero baudrate I believe we can, as
it appears setting baudrate registers is typically optional for UARTs.

We could also consider adding a log buffer which early init can write
errors and warnings to. After the console has been initialized (with
our best effort) we can then output all the errors and warnings from the
log buffer. If we had something like that, then we could add a warning
about the baudrate to that log along with warnings/errors for any other
DT parameters we fail to validate but attempt to work around.

Thanks,
drew

> 
> Regards,
> Xiang W
> > ---
> >  lib/utils/serial/gaisler-uart.c | 2 +-
> >  lib/utils/serial/shakti-uart.c  | 8 ++++++--
> >  lib/utils/serial/sifive-uart.c  | 2 +-
> >  lib/utils/serial/uart8250.c     | 7 +++++--
> >  4 files changed, 13 insertions(+), 6 deletions(-)
> > 
> > diff --git a/lib/utils/serial/gaisler-uart.c b/lib/utils/serial/gaisler-uart.c
> > index 5f30ee43b719..eec75cca0c98 100644
> > --- a/lib/utils/serial/gaisler-uart.c
> > +++ b/lib/utils/serial/gaisler-uart.c
> > @@ -70,7 +70,7 @@ int gaisler_uart_init(unsigned long base, u32 in_freq, u32 baudrate)
> >         uart_base = (volatile char *)base;
> >  
> >         /* Configure baudrate */
> > -       if (in_freq)
> > +       if (in_freq && baudrate)
> >                 set_reg(UART_REG_SCALER, in_freq / (baudrate * 8 + 7));
> >  
> >         ctrl = get_reg(UART_REG_CTRL);
> > diff --git a/lib/utils/serial/shakti-uart.c b/lib/utils/serial/shakti-uart.c
> > index 5f2fe75c9f33..2f09f141b9cf 100644
> > --- a/lib/utils/serial/shakti-uart.c
> > +++ b/lib/utils/serial/shakti-uart.c
> > @@ -47,8 +47,12 @@ static struct sbi_console_device shakti_console = {
> >  int shakti_uart_init(unsigned long base, u32 in_freq, u32 baudrate)
> >  {
> >         uart_base = (volatile char *)base;
> > -       u16 baud = (u16)(in_freq/(16 * baudrate));
> > -       writew(baud, uart_base + REG_BAUD);
> > +       u16 baud;
> > +
> > +       if (baudrate) {
> > +               baud = (u16)(in_freq / (16 * baudrate));
> > +               writew(baud, uart_base + REG_BAUD);
> > +       }
> >  
> >         sbi_console_set_device(&shakti_console);
> >  
> > diff --git a/lib/utils/serial/sifive-uart.c b/lib/utils/serial/sifive-uart.c
> > index 7078611a5274..3581d47a6d4b 100644
> > --- a/lib/utils/serial/sifive-uart.c
> > +++ b/lib/utils/serial/sifive-uart.c
> > @@ -97,7 +97,7 @@ int sifive_uart_init(unsigned long base, u32 in_freq, u32 baudrate)
> >         uart_baudrate = baudrate;
> >  
> >         /* Configure baudrate */
> > -       if (in_freq)
> > +       if (in_freq && baudrate)
> >                 set_reg(UART_REG_DIV, uart_min_clk_divisor(in_freq, baudrate));
> >  
> >         /* Disable interrupts */
> > diff --git a/lib/utils/serial/uart8250.c b/lib/utils/serial/uart8250.c
> > index 38ea11af4d31..99bf1bf733f2 100644
> > --- a/lib/utils/serial/uart8250.c
> > +++ b/lib/utils/serial/uart8250.c
> > @@ -93,7 +93,7 @@ static struct sbi_console_device uart8250_console = {
> >  int uart8250_init(unsigned long base, u32 in_freq, u32 baudrate, u32 reg_shift,
> >                   u32 reg_width, u32 reg_offset)
> >  {
> > -       u16 bdiv;
> > +       u16 bdiv = 0;
> >  
> >         uart8250_base      = (volatile char *)base + reg_offset;
> >         uart8250_reg_shift = reg_shift;
> > @@ -101,7 +101,10 @@ int uart8250_init(unsigned long base, u32 in_freq, u32 baudrate, u32 reg_shift,
> >         uart8250_in_freq   = in_freq;
> >         uart8250_baudrate  = baudrate;
> >  
> > -       bdiv = (uart8250_in_freq + 8 * uart8250_baudrate) / (16 * uart8250_baudrate);
> > +       if (uart8250_baudrate) {
> > +               bdiv = (uart8250_in_freq + 8 * uart8250_baudrate) /
> > +                      (16 * uart8250_baudrate);
> > +       }
> >  
> >         /* Disable all interrupts */
> >         set_reg(UART_IER_OFFSET, 0x00);
> > -- 
> > 2.36.1
> > 
> > 
> 
> 
> 
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