Re: [Xen-devel] [PATCH] ns16550: misc minor adjustments

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 12.11.15 at 12:07, <ian.campbell@xxxxxxxxxx> wrote:
> On Thu, 2015-11-12 at 03:31 -0700, Jan Beulich wrote:
>> --- a/docs/misc/xen-command-line.markdown
>> +++ b/docs/misc/xen-command-line.markdown
>> @@ -277,13 +277,13 @@ Flag to indicate whether to probe for a
>>  ACPI indicating none to be there.
>>  
>>  ### com1,com2
>> -> `= <baud>[/<clock_hz>][,[DPS][,[<io-base>|pci|amt][,[<irq>][,[<port-
>> bdf>][,[<bridge-bdf>]]]]]]`
>> +> `= <baud>[/<base_baud>][,[DPS][,[<io-base>|pci|amt][,[<irq>][,[<port-
>> bdf>][,[<bridge-bdf>]]]]]]`
>>  
>>  Both option `com1` and `com2` follow the same format.
>>  
>>  * `<baud>` may be either an integer baud rate, or the string `auto` if
>>    the bootloader or other earlier firmware has already set it up.
>> -* Optionally, a clock speed measured in hz can be specified.
>> +* Optionally, the base baud rate can be specified.
> 
> I do wonder if everyone trying to use this (which will include less-savy
> users who are trying to produce a useful bug report by our request) will
> know what this means?
> 
> On the other hand I'm not exactly sure how to better describe it other than
> as the "basic baud rate, i.e. the one you get with divisor == 1", which is
> no more likely to enlighten anyone who didn't already know that.
> 
> Is it true (enough) to say something like "typically the fastest baud rate
> supported by the device"?

Yes, I think so.

>> @@ -530,7 +531,8 @@ static void ns16550_setup_preirq(struct
>>          /* Baud rate already set: read it out from the divisor latch. */
>>          divisor  = ns_read_reg(uart, UART_DLL);
>>          divisor |= ns_read_reg(uart, UART_DLM) << 8;
>> -        uart->baud = uart->clock_hz / (divisor << 4);
>> +        if ( divisor )
>> +            uart->baud = uart->clock_hz / (divisor << 4);
> 
> Will following code cope with uart->baud == BAUD_AUTO? Or should we pick a
> static fallback rate (115200?) and set the divisor appropriately?

The device won't work with it left as BAUD_AUTO. Setting a guessed
baud rate alone won't help, as we'd then also have to write it into the
respective registers. And I don't think anyway we should do any
guessing here - if the command line option was wrong (regardless of
whether due to using auto or just a wrong baud rate), communication
won't work. All I think we should really care about is to not crash, the
more in a way not diagnosable over serial console.

Jan


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