Re: [PATCH 1/2] serial: tegra: Fix overflow in tegra_get_tolerance_rate()

[Svyatoslav Ryhel]

пн, 26 жовт. 2026 р. о 00:50 Patrick Fischer via B4 Relay
<devnull+admin.tsv-stahl-riesa.de@xxxxxxxxxx> пише:
>
> From: Patrick Fischer <admin@xxxxxxxxxxxxxxxxxx>
>
> tegra_get_tolerance_rate() raises or lowers the UART clock rate by the
> tolerance from "nvidia,adjust-baud-rates", given in units of 0.01 %. It
> calculates rate * tolerance in a long, which is only 32 bits wide on
> 32-bit ARM. With a tolerance of 136 this overflows from a clock rate of
> about 15.8 MHz, i.e. from about 1 Mbaud, and the rate ends up adjusted
> in the wrong direction.
>
> The Tegra Note 7 (Tegra114) uses a tolerance of 136 for 1 to 4 Mbaud on
> its Bluetooth UART, as in the downstream device tree. For 3 Mbaud the
> driver asks for 47.79 MHz instead of 48.65 MHz, the clock rounds that
> down to 408 MHz / 9 = 45.33 MHz (-5.6 %), and the WL1801 controller
> stops answering after the switch to 3 Mbaud (HCI commands time out
> with -110). The same overflow gives 23.31 MHz instead of 24 MHz at
> 1.5 Mbaud; both rates were confirmed on the device.
>
> Do the multiplication in 64 bits. With that the driver asks for
> 48.65 MHz at 3 Mbaud and gets 48.00 MHz.
>
> Tested on the Tegra Note 7 with the tolerance of 136: Bluetooth comes
> up at 3 Mbaud, scans and connects to a paired device, and the HCI
> counters show no errors.
>
> 64-bit platforms are not affected.
>
> Fixes: f04a3cc8d455 ("serial: tegra: add support to adjust baud rate")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Patrick Fischer <admin@xxxxxxxxxxxxxxxxxx>
> ---
>  drivers/tty/serial/serial-tegra.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>

Tested-by: Svyatoslav Ryhel <clamor95@xxxxxxxxx> # Tegratab (P1988) and LG P895
Reviewed-by: Svyatoslav Ryhel <clamor95@xxxxxxxxx>