| Message ID | 20200506233136.11842-2-Sergey.Semin@baikalelectronics.ru (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v3,1/4] serial: 8250: Fix max baud limit in generic 8250 port | expand |
On Thu, May 07, 2020 at 02:31:32AM +0300, Serge Semin wrote: > Standard 8250 UART ports are designed in a way so they can communicate > with baud rates up to 1/16 of a reference frequency. It's expected from > most of the currently supported UART controllers. That's why the former > version of serial8250_get_baud_rate() method called uart_get_baud_rate() > with min and max baud rates passed as (port->uartclk / 16 / UART_DIV_MAX) > and ((port->uartclk + tolerance) / 16) respectively. Doing otherwise, like > it was suggested in commit ("serial: 8250_mtk: support big baud rate."), > caused acceptance of bauds, which was higher than the normal UART > controllers actually supported. As a result if some user-space program > requested to set a baud greater than (uartclk / 16) it would have been > permitted without truncation, but then serial8250_get_divisor(baud) > (which calls uart_get_divisor() to get the reference clock divisor) would > have returned a zero divisor. Setting zero divisor will cause an > unpredictable effect varying from chip to chip. In case of DW APB UART the > communications just stop. > > Lets fix this problem by getting back the limitation of (uartclk + > tolerance) / 16 maximum baud supported by the generic 8250 port. Mediatek > 8250 UART ports driver developer shouldn't have touched it in the first > place notably seeing he already provided a custom version of set_termios() > callback in that glue-driver which took into account the extended baud > rate values and accordingly updated the standard and vendor-specific > divisor latch registers anyway. Some of the hardware support PS != 16 (8250_mid), but for now it lies to UART core for real UART clock because of its (core) hard cored assumption PS == 16 here and there. Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com> > > Fixes: 81bb549fdf14 ("serial: 8250_mtk: support big baud rate.") > Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru> > Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru> > Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de> > Cc: Paul Burton <paulburton@kernel.org> > Cc: Ralf Baechle <ralf@linux-mips.org> > Cc: Arnd Bergmann <arnd@arndb.de> > Cc: Long Cheng <long.cheng@mediatek.com> > Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com> > Cc: Maxime Ripard <mripard@kernel.org> > Cc: Catalin Marinas <catalin.marinas@arm.com> > Cc: Will Deacon <will@kernel.org> > Cc: Russell King <linux@armlinux.org.uk> > Cc: linux-mips@vger.kernel.org > Cc: linux-arm-kernel@lists.infradead.org > Cc: linux-mediatek@lists.infradead.org > --- > drivers/tty/serial/8250/8250_port.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c > index f77bf820b7a3..4d83c85a7389 100644 > --- a/drivers/tty/serial/8250/8250_port.c > +++ b/drivers/tty/serial/8250/8250_port.c > @@ -2615,6 +2615,8 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port, > struct ktermios *termios, > struct ktermios *old) > { > + unsigned int tolerance = port->uartclk / 100; > + > /* > * Ask the core to calculate the divisor for us. > * Allow 1% tolerance at the upper limit so uart clks marginally > @@ -2623,7 +2625,7 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port, > */ > return uart_get_baud_rate(port, termios, old, > port->uartclk / 16 / UART_DIV_MAX, > - port->uartclk); > + (port->uartclk + tolerance) / 16); > } > > void > -- > 2.25.1 >
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c index f77bf820b7a3..4d83c85a7389 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -2615,6 +2615,8 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port, struct ktermios *termios, struct ktermios *old) { + unsigned int tolerance = port->uartclk / 100; + /* * Ask the core to calculate the divisor for us. * Allow 1% tolerance at the upper limit so uart clks marginally @@ -2623,7 +2625,7 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port, */ return uart_get_baud_rate(port, termios, old, port->uartclk / 16 / UART_DIV_MAX, - port->uartclk); + (port->uartclk + tolerance) / 16); } void
Standard 8250 UART ports are designed in a way so they can communicate with baud rates up to 1/16 of a reference frequency. It's expected from most of the currently supported UART controllers. That's why the former version of serial8250_get_baud_rate() method called uart_get_baud_rate() with min and max baud rates passed as (port->uartclk / 16 / UART_DIV_MAX) and ((port->uartclk + tolerance) / 16) respectively. Doing otherwise, like it was suggested in commit ("serial: 8250_mtk: support big baud rate."), caused acceptance of bauds, which was higher than the normal UART controllers actually supported. As a result if some user-space program requested to set a baud greater than (uartclk / 16) it would have been permitted without truncation, but then serial8250_get_divisor(baud) (which calls uart_get_divisor() to get the reference clock divisor) would have returned a zero divisor. Setting zero divisor will cause an unpredictable effect varying from chip to chip. In case of DW APB UART the communications just stop. Lets fix this problem by getting back the limitation of (uartclk + tolerance) / 16 maximum baud supported by the generic 8250 port. Mediatek 8250 UART ports driver developer shouldn't have touched it in the first place notably seeing he already provided a custom version of set_termios() callback in that glue-driver which took into account the extended baud rate values and accordingly updated the standard and vendor-specific divisor latch registers anyway. Fixes: 81bb549fdf14 ("serial: 8250_mtk: support big baud rate.") Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru> Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru> Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de> Cc: Paul Burton <paulburton@kernel.org> Cc: Ralf Baechle <ralf@linux-mips.org> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Long Cheng <long.cheng@mediatek.com> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com> Cc: Maxime Ripard <mripard@kernel.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Will Deacon <will@kernel.org> Cc: Russell King <linux@armlinux.org.uk> Cc: linux-mips@vger.kernel.org Cc: linux-arm-kernel@lists.infradead.org Cc: linux-mediatek@lists.infradead.org --- drivers/tty/serial/8250/8250_port.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)