On Thu, Nov 10, 2016 at 09:56:41AM +0100, Thomas Gleixner wrote:
> On Thu, 10 Nov 2016, Lu Baolu wrote:
> > This seems to be a common issue for all early printk drivers.
>
> No. The other early printk drivers like serial do not have that problem as
> they simply do:
>
> while (*buf) {
> while (inb(UART) & TX_BUSY)
> cpu_relax();
> outb(*buf++, UART);
> }
Right, which is why actual UARTs rule. If only laptops still had pinouts
for them life would be sooooo much better.
Ideally the USB debug port would be a virtual UART and its interface as
simple and robust.
> The wait for the UART to become ready is independent of the context as it
> solely depends on the hardware.
>
> As a result you can see the output from irq/nmi intermingled with the one
> from thread context, but that's the only problem they have.
>
> The only thing you can do to make this work is to prevent printing in NMI
> context:
>
> write()
> {
> if (in_nmi())
> return;
>
> raw_spinlock_irqsave(&lock, flags);
> ....
>
> That fully serializes the writes and just ignores NMI context printks. Not
> optimal, but I fear that's all you can do.
I would also suggest telling the hardware people they have designed
something near the brink of useless. If you cannot do random exception
context debugging (#DB, #NMI, #MCE etc..) then there's a whole host of
problems that simply cannot be debugged.
Also note that kdb runs from NMI context, so you'll not be able to
support that either.
