[U-Boot] [PATCH 5/6] serial: ns16550: fix debug uart putc called before init

[Marek Vasut]

On 08/10/2018 07:22 AM, Simon Goldschmidt wrote:
> On 10.08.2018 00:41, Marek Vasut wrote:
>> On 08/10/2018 12:35 AM, Andy Shevchenko wrote:
>>> On Fri, Aug 10, 2018 at 12:45 AM, Marek Vasut <marex at denx.de> wrote:
>>>> On 08/09/2018 11:13 PM, Adam Ford wrote:
>>>>> On Thu, Aug 9, 2018 at 2:08 PM Simon Goldschmidt
>>>>> <simon.k.r.goldschmidt at gmail.com> wrote:
>>>>>> If _debug_uart_putc() is called before _debug_uart_init(), the
>>>>>> ns16550 debug uart driver hangs in a tight loop waiting for the
>>>>>> tx FIFO to get empty.
>>>>>>
>>>>>> As this can happen via a printf sneaking in before the port calls
>>>>>> debug_uart_init(), let's rather ignore characters before the debug
>>>>>> uart is initialized.
>>>>>>
>>>>>> This is done by reading the baudrate divisor and aborting if is zero.
>>>>>>   static inline void _debug_uart_putc(int ch)
>>>>>>   {
>>>>>>          struct NS16550 *com_port = (struct NS16550
>>>>>> *)CONFIG_DEBUG_UART_BASE;
>>>>>> +       while (!(serial_din(&com_port->lsr) & UART_LSR_THRE)) {
>>>>>> +               if (!NS16550_read_baud_divisor(com_port))
>>>>> Unless there is a change that the read_baud_divisor will change while
>>>>> we're waiting for the character, could we move this check before the
>>>>> while statement?  This would reduce the check for the divisor to 1x
>>>>> and the while statement would only have one comparison to do.  I
>>>>> realize it's rather trivial, but the way I see it, there is no reason
>>>>> to do the while statement at all if the read_baud_divisor fails and
>>>>> there if there is a baud divisor, we should only need to check it
>>>>> once.
>>>> This looks like a massive hack -- what about having a flag which says
>>>> that the debug uart was/was not inited somewhere ?
>>> Agree, why not to cache divisor value, for example, instead of doing
>>> slow I/O?
>> But why do we care about the divisor at all ?
> 
> Because if the divisor is zero, the UART is disabled.
> 
>> The real problem I believe
>> is that someone can call debug UART print/read functions before it is
>> inited.
>>
> I know this is a hack. I did it like that because I need something like
> this to get debug uart to work on socfpga gen5 (there always is a printf
> before debug uart init is possible).
> 
> A generic solution for all debug uarts would be better of course, but
> given the point in SPL runtime, we might have to add a field to 'gd' for
> that, or does a global variable work at that point already?

GD field might be needed indeed.

-- 
Best regards,
Marek Vasut