[U-Boot-Users] [PATCH] Round the serial port clock divisor value returned by calc_divisor()

[Hugo Villeneuve]

Jerry Van Baren wrote:
> Hugo Villeneuve wrote:
>> Round the serial port clock divisor value returned by calc_divisor().
>> 
>> Signed-off-by: Hugo Villeneuve <hugo.villeneuve at lyrtech.com>
>> 
>> ---
>> 
>> Rounding is important, especially when using high baud rates
>> values like 115200bps. When using the non-rounded value, some
>> boards will work and some won't.
>> 
>>  drivers/serial/serial.c |    7 ++++++-
>>  1 files changed, 6 insertions(+), 1 deletions(-)
>> 
>> diff --git a/drivers/serial/serial.c b/drivers/serial/serial.c index
>> 76425d8..7e315ad 100644 --- a/drivers/serial/serial.c
>> +++ b/drivers/serial/serial.c
>> @@ -124,6 +124,8 @@ static NS16550_t serial_ports[4] = {
>> 
>>  static int calc_divisor (NS16550_t port)
>>  {
>> +	u32 divisor_x10;
>> +
>>  #ifdef CONFIG_OMAP1510
>>  	/* If can't cleanly clock 115200 set div to 1 */
>>  	if ((CFG_NS16550_CLK == 12000000) && (gd->baudrate == 115200)) {
>> @@ -144,8 +146,11 @@ static int calc_divisor (NS16550_t port)  #else
>>  #define MODE_X_DIV 16
>>  #endif
>> -	return (CFG_NS16550_CLK / MODE_X_DIV / gd->baudrate);
>> 
>> +	/* Compute divisor value with rounding by adding 0.5 */
>> +	divisor_x10 = (10 * CFG_NS16550_CLK) / MODE_X_DIV / gd->baudrate; +
>> +	return (divisor_x10 + 5) / 10;
>>  }
>> 
>>  #if !defined(CONFIG_SERIAL_MULTI)
> 
> Hi Hugo,
> 
> Will a real rounding work?  Work better?  If I got my mental math and
> parenthesis right and the resulting math doesn't overflow your
> registers, the following will add 1/2 the baud rate scaled by the
> MODE_X_DIV and then perform the divide which will do full rounding.
> 
> 	return (((CFG_NS16550_CLK + ((gd->baudrate / 2)* MODE_X_DIV))
> 		/ MODE_X_DIV) / gd->baudrate);
I tested it and it works for me.
But maybe the code is less obvious that way?
 
> Alternately, I prefer to scale up by 16 and then divide by 8 since
> processors can do that very efficiently ( << 4 followed by >> 3).
Right.

I prefer the second solution for its simplicity, but the first one also works.

Hugo.

Hugo Villeneuve
Hardware developer | Concepteur matériel
Lyrtech
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