[Skiboot] [PATCH 1/3] uart: Drop console write data if BMC becomes unresponsive

[Vasant Hegde]

On 5/20/20 6:32 PM, Nicholas Piggin wrote:
> Excerpts from Vasant Hegde's message of May 20, 2020 9:51 pm:
>> If BMC becomes unresponsive (ex: during BMC reboot) during console write
>> then we may get stuck in uart_wait_tx_room(). This will result in kernel
>> lockups and in some cases host becomes unresponsive.
>>
>> This patch retries for `tx_room` for predefined number of time. If we don't
>> get space within that time it will return zero. This will result in
>> dropping console message.
> 
> Is the number of times important, or should it be time based?

It can be time base as well. May be I will introduce timeout option in v2.
> 
>>
>> Signed-off-by: Vasant Hegde <hegdevasant at linux.vnet.ibm.com>
>> ---
>>   hw/lpc-uart.c | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/hw/lpc-uart.c b/hw/lpc-uart.c
>> index 979a617c3..ed56a4bdc 100644
>> --- a/hw/lpc-uart.c
>> +++ b/hw/lpc-uart.c
>> @@ -63,6 +63,8 @@ static void *mmio_uart_base;
>>   static int uart_console_policy = UART_CONSOLE_OPAL;
>>   static int lpc_irq = -1;
>>   
>> +#define UART_WRITE_MAX_RETRIES	0x100
>> +
>>   void uart_set_console_policy(int policy)
>>   {
>>   	uart_console_policy = policy;
>> @@ -106,6 +108,8 @@ static void uart_check_tx_room(void)
>>   
>>   static void uart_wait_tx_room(void)
>>   {
>> +	int retry_cnt = 0;
>> +
>>   	while (!tx_room) {
>>   		uart_check_tx_room();
>>   		if (!tx_room) {
>> @@ -113,6 +117,10 @@ static void uart_wait_tx_room(void)
>>   			do {
>>   				barrier();
>>   				uart_check_tx_room();
>> +
>> +				retry_cnt++;
>> +				if (retry_cnt > UART_WRITE_MAX_RETRIES)
>> +					return;
>>   			} while (!tx_room);
>>   			smt_medium();
> 
> Must restore SMT priority to medium.

Oh yes. I missed it. Will fix.

Thanks!

-Vasant