Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability

[Peter Zijlstra]

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.