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

[sashiko-bot@kernel.org]

> From: Patrick Fischer <admin@tsv-stahl-riesa.de>
> 
> 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@vger.kernel.org
> Signed-off-by: Patrick Fischer <admin@tsv-stahl-riesa.de>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261025-upstream-serial-tegra-overflow-v1-0-267b5b7a79ee@tsv-stahl-riesa.de?part=1