| Message ID | 20220924102718.2984-2-pali@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | ftdi_sio driver improvements | expand |
On Sat, Sep 24, 2022 at 12:27:12PM +0200, Pali Rohár wrote: > 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. As I already commented on v2: 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. > > 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> And as I mentioned the other week, this SoB change is not correct as it is you who are submitting these patches now and it's not clear whether you intended that Marek should be attributed as co-author or not. Johan
diff --git a/drivers/usb/serial/ftdi_sio.c b/drivers/usb/serial/ftdi_sio.c index 52d59be92034..b2febe8d573a 100644 --- a/drivers/usb/serial/ftdi_sio.c +++ b/drivers/usb/serial/ftdi_sio.c @@ -1162,6 +1162,8 @@ 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) @@ -1186,6 +1188,8 @@ 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. */ @@ -1209,6 +1213,8 @@ 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;
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(+)