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

[Simon Goldschmidt]

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?


Simon