On 29. 04. 24, 15:47, Hugo Villeneuve wrote:
On Mon, 29 Apr 2024 08:39:22 +0200
Jiri Slaby <jirislaby@xxxxxxxxxx> wrote:
On 26. 04. 24, 15:59, Hugo Villeneuve wrote:
From: Hugo Villeneuve <hvilleneuve@xxxxxxxxxxxx>
When using a high speed clock with a low baud rate, the 4x prescaler is
automatically selected if required. In that case, sc16is7xx_set_baud()
properly configures the chip registers, but returns an incorrect baud
rate by not taking into account the prescaler value. This incorrect baud
rate is then fed to uart_update_timeout().
For example, with an input clock of 80MHz, and a selected baud rate of 50,
sc16is7xx_set_baud() will return 200 instead of 50.
Fix this by first changing the prescaler variable to hold the selected
prescaler value instead of the MCR bitfield. Then properly take into
account the selected prescaler value in the return value computation.
Also add better documentation about the divisor value computation.
Fixes: dfeae619d781 ("serial: sc16is7xx")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Hugo Villeneuve <hvilleneuve@xxxxxxxxxxxx>
---
drivers/tty/serial/sc16is7xx.c | 23 ++++++++++++++++++-----
1 file changed, 18 insertions(+), 5 deletions(-)
diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
index 03cf30e20b75..dcd6c5615401 100644
--- a/drivers/tty/serial/sc16is7xx.c
+++ b/drivers/tty/serial/sc16is7xx.c
@@ -555,16 +555,28 @@ static bool sc16is7xx_regmap_noinc(struct device *dev, unsigned int reg)
return reg == SC16IS7XX_RHR_REG;
}
+/*
+ * Configure programmable baud rate generator (divisor) according to the
+ * desired baud rate.
+ *
+ * From the datasheet, the divisor is computed according to:
+ *
+ * XTAL1 input frequency
+ * -----------------------
+ * prescaler
+ * divisor = ---------------------------
+ * baud-rate x sampling-rate
+ */
static int sc16is7xx_set_baud(struct uart_port *port, int baud)
{
struct sc16is7xx_one *one = to_sc16is7xx_one(port, port);
u8 lcr;
- u8 prescaler = 0;
+ int prescaler = 1;
Ugh, why do you move to signed arithmetics?
Hi Jiri,
before this patch, the variable prescaler was used to store an 8 bit
bitfield. Now the variable meaning is changed to be used as the
prescaler value, which can be 1 or 4 in this case. Leaving
it as u8 would still be ok, or making it "unsigned int" maybe?
Both :). What you prefer -- uint matches more IMO, given it's now a
value and not a register...
thanks,
--
js
suse labs
