[PATCH v2 2/4] serial: 8250_dw: Only accept a contiguous DLF readback

[Vinicius Pedrosa <vinicius.eduardo.pedrosa@gmail.com>]

dw8250_setup_port() writes all ones to offset 0xc0 and treats any
nonzero readback as the DLF register. DLF is a bit field starting at
bit 0, so a real one reads back as a contiguous 2^n - 1 mask. On some
Allwinner SoCs offset 0xc0 is the RS485 control register instead
(UART_485_CTL in the A733 User Manual), whose implemented bits are not
contiguous. The probe still takes its readback for a DLF of fls(readback)
bits, and every later divisor change writes the fractional divisor into
the RS485 control register.

Only accept a readback of the form 2^n - 1.

The probe itself is unchanged: it still writes all ones to offset 0xc0
and then restores the old value, so this only changes how the readback
is interpreted.

The A733 reads back 0x19f, and Andre Przywara observed 0x9f on the
A523. Without this check the A733 RS485 control register reads 0xb after
boot; with it, the register stays 0 across 300 termios toggle and
restore cycles.

Suggested-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Vinicius Pedrosa <vinicius.eduardo.pedrosa@gmail.com>
---
 drivers/tty/serial/8250/8250_dwlib.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_dwlib.c b/drivers/tty/serial/8250/8250_dwlib.c
index 325793f20256..1b0192efe162 100644
--- a/drivers/tty/serial/8250/8250_dwlib.c
+++ b/drivers/tty/serial/8250/8250_dwlib.c
@@ -225,7 +225,11 @@ void dw8250_setup_port(struct uart_port *p)
 	reg = dw8250_readl_ext(p, DW_UART_DLF);
 	dw8250_writel_ext(p, DW_UART_DLF, old_dlf);
 
-	if (reg) {
+	/*
+	 * DLF is a bit field starting at bit 0, so a real one reads back as
+	 * 2^n - 1. Any other value means this offset is something else.
+	 */
+	if (reg && !(reg & (reg + 1))) {
 		pd->dlf_size = fls(reg);
 		p->get_divisor = dw8250_get_divisor;
 		/* Keep a hook the driver has already installed */