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

[Simon Goldschmidt]

On 10.08.2018 11:51, Marek Vasut wrote:
> 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.
>
Right. I'll drop this patch in the next version of the series and 
instead I'll try to work out something that works for all debug uarts 
drivers using a gd field.


Thanks,

Simon