[PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio

[PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio

[Alexey Brodkin]

Modify divisor to select the nearest baud rate divider rather than the
lowest. It minimizes baud rate errors especially on low UART clock
frequencies.

For example, if uartclk is 33000000 and baud is 115200 the ratio is
about 17.9 The current code selects 17 (5% error) but should select 18
(0.5% error).

On the same lines as following:
http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.9-rc3/2.6.9-rc3-mm2/broken-out/serial-pick-nearest-baud-rate-divider.patch

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
---
 drivers/tty/serial/8250_early.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250_early.c b/drivers/tty/serial/8250_early.c
index eaafb98..cfc46b7 100644
--- a/drivers/tty/serial/8250_early.c
+++ b/drivers/tty/serial/8250_early.c
@@ -140,7 +140,7 @@ static void __init init_port(struct early_serial8250_device *device)
 	serial_out(port, UART_FCR, 0);		/* no fifo */
 	serial_out(port, UART_MCR, 0x3);	/* DTR + RTS */
 
-	divisor = port->uartclk / (16 * device->baud);
+	divisor = (port->uartclk + (8 * device->baud)) / (16 *	device->baud);
 	c = serial_in(port, UART_LCR);
 	serial_out(port, UART_LCR, c | UART_LCR_DLAB);
 	serial_out(port, UART_DLL, divisor & 0xff);
-- 
1.7.9.5

[PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio

[Alexey Brodkin]

Modify divisor to select the nearest baud rate divider rather than the
lowest. It minimizes baud rate errors especially on low UART clock
frequencies.

For example, if uartclk is 33000000 and baud is 115200 the ratio is
about 17.9 The current code selects 17 (5% error) but should select 18
(0.5% error).

On the same lines as following:
http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.9-rc3/2.6.9-rc3-mm2/broken-out/serial-pick-nearest-baud-rate-divider.patch

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
---
 drivers/tty/serial/8250_early.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250_early.c b/drivers/tty/serial/8250_early.c
index eaafb98..cfc46b7 100644
--- a/drivers/tty/serial/8250_early.c
+++ b/drivers/tty/serial/8250_early.c
@@ -140,7 +140,7 @@ static void __init init_port(struct early_serial8250_device *device)
 	serial_out(port, UART_FCR, 0);		/* no fifo */
 	serial_out(port, UART_MCR, 0x3);	/* DTR + RTS */
 
-	divisor = port->uartclk / (16 * device->baud);
+	divisor = (port->uartclk + (8 * device->baud)) / (16 *	device->baud);
 	c = serial_in(port, UART_LCR);
 	serial_out(port, UART_LCR, c | UART_LCR_DLAB);
 	serial_out(port, UART_DLL, divisor & 0xff);
-- 
1.7.9.5

Re: [PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio

[Jiri Slaby]

On 09/28/2012 02:03 PM, Alexey Brodkin wrote:
> Modify divisor to select the nearest baud rate divider rather than the
> lowest. It minimizes baud rate errors especially on low UART clock
> frequencies.
> 
> For example, if uartclk is 33000000 and baud is 115200 the ratio is
> about 17.9 The current code selects 17 (5% error) but should select 18
> (0.5% error).
> 
> On the same lines as following:
> http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.9-rc3/2.6.9-rc3-mm2/broken-out/serial-pick-nearest-baud-rate-divider.patch
> 
> Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
> ---
>  drivers/tty/serial/8250_early.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/8250_early.c b/drivers/tty/serial/8250_early.c
> index eaafb98..cfc46b7 100644
> --- a/drivers/tty/serial/8250_early.c
> +++ b/drivers/tty/serial/8250_early.c
> @@ -140,7 +140,7 @@ static void __init init_port(struct early_serial8250_device *device)
>  	serial_out(port, UART_FCR, 0);		/* no fifo */
>  	serial_out(port, UART_MCR, 0x3);	/* DTR + RTS */
>  
> -	divisor = port->uartclk / (16 * device->baud);
> +	divisor = (port->uartclk + (8 * device->baud)) / (16 *	device->baud);

So this should be in fact DIV_ROUND_CLOSEST(), right?

But anyway I'm missing any explanation of *why* this is needed? This
should be part of the commit log. Is there any bug you are fixing here?

thanks,
-- 
js
suse labs

Re: [PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio

[Jiri Slaby]

On 09/28/2012 02:19 PM, Alexey Brodkin wrote:
> Modify divisor to select the nearest baud rate divider rather than the
> lowest. It minimizes baud rate errors especially on low UART clock
> frequencies.
> 
> For example, if uartclk is 33000000 and baud is 115200 the ratio is
> about 17.9 The current code selects 17 (5% error) but should select 18
> (0.5% error).
> 
> On the same lines as following:
> http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.9-rc3/2.6.9-rc3-mm2/broken-out/serial-pick-nearest-baud-rate-divider.patch
> 
> Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
> ---
>  drivers/tty/serial/8250_early.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/8250_early.c b/drivers/tty/serial/8250_early.c
> index eaafb98..cfc46b7 100644
> --- a/drivers/tty/serial/8250_early.c
> +++ b/drivers/tty/serial/8250_early.c
> @@ -140,7 +140,7 @@ static void __init init_port(struct early_serial8250_device *device)
>  	serial_out(port, UART_FCR, 0);		/* no fifo */
>  	serial_out(port, UART_MCR, 0x3);	/* DTR + RTS */
>  
> -	divisor = port->uartclk / (16 * device->baud);
> +	divisor = (port->uartclk + (8 * device->baud)) / (16 *	device->baud);

So this should be in fact DIV_ROUND_CLOSEST(), right?

But anyway I'm missing any explanation of *why* this is needed? This
should be part of the commit log. Is there any bug you are fixing here?

thanks,
-- 
js
suse labs

Re: [PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio

[Alan Cox]

On Fri, 28 Sep 2012 16:19:08 +0400
Alexey Brodkin <Alexey.Brodkin@synopsys.com> wrote:

> Modify divisor to select the nearest baud rate divider rather than the
> lowest. It minimizes baud rate errors especially on low UART clock
> frequencies.
> 
> For example, if uartclk is 33000000 and baud is 115200 the ratio is
> about 17.9 The current code selects 17 (5% error) but should select 18
> (0.5% error).
> 
> On the same lines as following:
> http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.9-rc3/2.6.9-rc3-mm2/broken-out/serial-pick-nearest-baud-rate-divider.patch
> 
> Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

Seems sensible to me.

Acked-by: Alan Cox <alan@linux.intel.com>

Re: [PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio

[Vineet Gupta]

On Friday 28 September 2012 06:32 PM, Jiri Slaby wrote:
> On 09/28/2012 02:19 PM, Alexey Brodkin wrote:
>> Modify divisor to select the nearest baud rate divider rather than the
>> lowest. It minimizes baud rate errors especially on low UART clock
>> frequencies.
>>
>> For example, if uartclk is 33000000 and baud is 115200 the ratio is
>> about 17.9 The current code selects 17 (5% error) but should select 18
>> (0.5% error).
>>
>> On the same lines as following:
>> http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.9-rc3/2.6.9-rc3-mm2/broken-out/serial-pick-nearest-baud-rate-divider.patch
>>
>> Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
>> ---
>>  drivers/tty/serial/8250_early.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/tty/serial/8250_early.c b/drivers/tty/serial/8250_early.c
>> index eaafb98..cfc46b7 100644
>> --- a/drivers/tty/serial/8250_early.c
>> +++ b/drivers/tty/serial/8250_early.c
>> @@ -140,7 +140,7 @@ static void __init init_port(struct early_serial8250_device *device)
>>  	serial_out(port, UART_FCR, 0);		/* no fifo */
>>  	serial_out(port, UART_MCR, 0x3);	/* DTR + RTS */
>>  
>> -	divisor = port->uartclk / (16 * device->baud);
>> +	divisor = (port->uartclk + (8 * device->baud)) / (16 *	device->baud);
> So this should be in fact DIV_ROUND_CLOSEST(), right?
>
> But anyway I'm missing any explanation of *why* this is needed? This
> should be part of the commit log. Is there any bug you are fixing here?

Actually the issue showed up when using the stock 8250 driver for
Synopsys DW UART. This was on a FPGA with ~50MHz clk. When we enabled
early serial, we saw garbage which Alexey narrowed down to the rounding
error. So the bug had been latent and it only showed up with such low
clk rates.

Thx,
Vineet

Re: [PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio

[Jiri Slaby]

On 09/29/2012 08:40 AM, Vineet Gupta wrote:
> On Friday 28 September 2012 06:32 PM, Jiri Slaby wrote:
>> On 09/28/2012 02:19 PM, Alexey Brodkin wrote:
>>> Modify divisor to select the nearest baud rate divider rather than the
>>> lowest. It minimizes baud rate errors especially on low UART clock
>>> frequencies.
>>>
>>> For example, if uartclk is 33000000 and baud is 115200 the ratio is
>>> about 17.9 The current code selects 17 (5% error) but should select 18
>>> (0.5% error).
>>>
>>> On the same lines as following:
>>> http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.9-rc3/2.6.9-rc3-mm2/broken-out/serial-pick-nearest-baud-rate-divider.patch
>>>
>>> Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
>>> ---
>>>  drivers/tty/serial/8250_early.c |    2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/tty/serial/8250_early.c b/drivers/tty/serial/8250_early.c
>>> index eaafb98..cfc46b7 100644
>>> --- a/drivers/tty/serial/8250_early.c
>>> +++ b/drivers/tty/serial/8250_early.c
>>> @@ -140,7 +140,7 @@ static void __init init_port(struct early_serial8250_device *device)
>>>  	serial_out(port, UART_FCR, 0);		/* no fifo */
>>>  	serial_out(port, UART_MCR, 0x3);	/* DTR + RTS */
>>>  
>>> -	divisor = port->uartclk / (16 * device->baud);
>>> +	divisor = (port->uartclk + (8 * device->baud)) / (16 *	device->baud);
>> So this should be in fact DIV_ROUND_CLOSEST(), right?
>>
>> But anyway I'm missing any explanation of *why* this is needed? This
>> should be part of the commit log. Is there any bug you are fixing here?
> 
> Actually the issue showed up when using the stock 8250 driver for
> Synopsys DW UART. This was on a FPGA with ~50MHz clk. When we enabled
> early serial, we saw garbage which Alexey narrowed down to the rounding
> error. So the bug had been latent and it only showed up with such low
> clk rates.

Yes, this is a perfect argument I was asking for and should have been a
part of the commit...

thanks,
-- 
js
suse labs

Re: [PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio

[Vineet Gupta]

On Saturday 29 September 2012 12:37 PM, Jiri Slaby wrote:
> On 09/29/2012 08:40 AM, Vineet Gupta wrote:
>> On Friday 28 September 2012 06:32 PM, Jiri Slaby wrote:
>>> On 09/28/2012 02:19 PM, Alexey Brodkin wrote:
>>>> Modify divisor to select the nearest baud rate divider rather than the
>>>> lowest. It minimizes baud rate errors especially on low UART clock
>>>> frequencies.
>>>>
>>>> For example, if uartclk is 33000000 and baud is 115200 the ratio is
>>>> about 17.9 The current code selects 17 (5% error) but should select 18
>>>> (0.5% error).
>>>>
>>>> On the same lines as following:
>>>> http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.9-rc3/2.6.9-rc3-mm2/broken-out/serial-pick-nearest-baud-rate-divider.patch
>>>>
>>>> Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
>>>> ---
>>>>  drivers/tty/serial/8250_early.c |    2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/tty/serial/8250_early.c b/drivers/tty/serial/8250_early.c
>>>> index eaafb98..cfc46b7 100644
>>>> --- a/drivers/tty/serial/8250_early.c
>>>> +++ b/drivers/tty/serial/8250_early.c
>>>> @@ -140,7 +140,7 @@ static void __init init_port(struct early_serial8250_device *device)
>>>>  	serial_out(port, UART_FCR, 0);		/* no fifo */
>>>>  	serial_out(port, UART_MCR, 0x3);	/* DTR + RTS */
>>>>  
>>>> -	divisor = port->uartclk / (16 * device->baud);
>>>> +	divisor = (port->uartclk + (8 * device->baud)) / (16 *	device->baud);
>>> So this should be in fact DIV_ROUND_CLOSEST(), right?
>>>
>>> But anyway I'm missing any explanation of *why* this is needed? This
>>> should be part of the commit log. Is there any bug you are fixing here?
>> Actually the issue showed up when using the stock 8250 driver for
>> Synopsys DW UART. This was on a FPGA with ~50MHz clk. When we enabled
>> early serial, we saw garbage which Alexey narrowed down to the rounding
>> error. So the bug had been latent and it only showed up with such low
>> clk rates.
> Yes, this is a perfect argument I was asking for and should have been a
> part of the commit...

First timer's anxiety I suppose ! Per your DIV_ROUND_CLOSEST()
suggestion, Alexey has already sent out a v2 patch. I presume he needs
to respin a v3 to beef up the commitlog along the lines of above !

Thx,
-Vineet