diff mbox series

[v2,1/7] USB: serial: ftdi_sio: Fix divisor overflow

Message ID 20220712115306.26471-2-kabel@kernel.org
State New
Headers show
Series ftdi_sio driver improvements | expand

Commit Message

Marek Behún July 12, 2022, 11:53 a.m. UTC 
From: Pali Rohár <pali@kernel.org>

The baud rate generating divisor is a 17-bit wide (14 bits integer part
and 3 bits fractional part).

Example:
  base clock = 48 MHz
  requested baud rate = 180 Baud
  divisor = 48,000,000 / (16 * 180) = 0b100000100011010.101

  In this case the MSB gets discarded because of the overflow, and the
  actually used divisor will be 0b100011010.101 = 282.625, resulting
  in baud rate 10615 Baud, instead of the requested 180 Baud.

The best possible thing to do in this case is to generate lowest possible
baud rate (in the example it would be 183 Baud), by using maximum possible
divisor.

In case of divisor overflow, use maximum possible divisor.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Pali Rohár <pali@kernel.org>
Tested-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Cc: stable@vger.kernel.org
---
 drivers/usb/serial/ftdi_sio.c | 6 ++++++
 1 file changed, 6 insertions(+)

Comments

Johan Hovold July 24, 2022, 12:02 p.m. UTC | #1 
On Tue, Jul 12, 2022 at 01:53:00PM +0200, Marek Behún wrote:
> From: Pali Rohár <pali@kernel.org>
> 
> The baud rate generating divisor is a 17-bit wide (14 bits integer part
> and 3 bits fractional part).
> 
> Example:
>   base clock = 48 MHz
>   requested baud rate = 180 Baud
>   divisor = 48,000,000 / (16 * 180) = 0b100000100011010.101
> 
>   In this case the MSB gets discarded because of the overflow, and the
>   actually used divisor will be 0b100011010.101 = 282.625, resulting
>   in baud rate 10615 Baud, instead of the requested 180 Baud.
> 
> The best possible thing to do in this case is to generate lowest possible
> baud rate (in the example it would be 183 Baud), by using maximum possible
> divisor.

Actually, the best way to handle this is to add a sanity check for the
lowest supported check as you do in the next patch. That one makes this
change superfluous.

> In case of divisor overflow, use maximum possible divisor.

Johan
diff mbox series

Patch

diff --git a/drivers/usb/serial/ftdi_sio.c b/drivers/usb/serial/ftdi_sio.c
index b440d338a895..ea40f367e70c 100644
--- a/drivers/usb/serial/ftdi_sio.c
+++ b/drivers/usb/serial/ftdi_sio.c
@@ -1157,6 +1157,8 @@  static unsigned short int ftdi_232am_baud_base_to_divisor(int baud, int base)
 	int divisor3 = DIV_ROUND_CLOSEST(base, 2 * baud);
 	if ((divisor3 & 0x7) == 7)
 		divisor3++; /* round x.7/8 up to x+1 */
+	if (divisor3 > GENMASK(16, 0))
+		divisor3 = GENMASK(16, 0);
 	divisor = divisor3 >> 3;
 	divisor3 &= 0x7;
 	if (divisor3 == 1)
@@ -1181,6 +1183,8 @@  static u32 ftdi_232bm_baud_base_to_divisor(int baud, int base)
 	u32 divisor;
 	/* divisor shifted 3 bits to the left */
 	int divisor3 = DIV_ROUND_CLOSEST(base, 2 * baud);
+	if (divisor3 > GENMASK(16, 0))
+		divisor3 = GENMASK(16, 0);
 	divisor = divisor3 >> 3;
 	divisor |= (u32)divfrac[divisor3 & 0x7] << 14;
 	/* Deal with special cases for highest baud rates. */
@@ -1204,6 +1208,8 @@  static u32 ftdi_2232h_baud_base_to_divisor(int baud, int base)
 
 	/* hi-speed baud rate is 10-bit sampling instead of 16-bit */
 	divisor3 = DIV_ROUND_CLOSEST(8 * base, 10 * baud);
+	if (divisor3 > GENMASK(16, 0))
+		divisor3 = GENMASK(16, 0);
 
 	divisor = divisor3 >> 3;
 	divisor |= (u32)divfrac[divisor3 & 0x7] << 14;