lib8390: comment on locking by Alan Cox

Additional explanation of problems with locking by Alan Cox.
Signed-off-by: Jarek Poplawski <jarkao2@o2.pl>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Paul Gortmaker <p_gortmaker@yahoo.com>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/net/lib8390.c

@@ -143,6 +143,52 @@ static void __NS8390_init(struct net_device *dev, int startp);
  *	annoying the transmit function is called bh atomic. That places
  *	restrictions on the user context callers as disable_irq won't save
  *	them.
+ *
+ *	Additional explanation of problems with locking by Alan Cox:
+ *
+ *	"The author (me) didn't use spin_lock_irqsave because the slowness of the
+ *	card means that approach caused horrible problems like losing serial data
+ *	at 38400 baud on some chips. Rememeber many 8390 nics on PCI were ISA
+ *	chips with FPGA front ends.
+ *	
+ *	Ok the logic behind the 8390 is very simple:
+ *	
+ *	Things to know
+ *		- IRQ delivery is asynchronous to the PCI bus
+ *		- Blocking the local CPU IRQ via spin locks was too slow
+ *		- The chip has register windows needing locking work
+ *	
+ *	So the path was once (I say once as people appear to have changed it
+ *	in the mean time and it now looks rather bogus if the changes to use
+ *	disable_irq_nosync_irqsave are disabling the local IRQ)
+ *	
+ *	
+ *		Take the page lock
+ *		Mask the IRQ on chip
+ *		Disable the IRQ (but not mask locally- someone seems to have
+ *			broken this with the lock validator stuff)
+ *			[This must be _nosync as the page lock may otherwise
+ *				deadlock us]
+ *		Drop the page lock and turn IRQs back on
+ *		
+ *		At this point an existing IRQ may still be running but we can't
+ *		get a new one
+ *	
+ *		Take the lock (so we know the IRQ has terminated) but don't mask
+ *	the IRQs on the processor
+ *		Set irqlock [for debug]
+ *	
+ *		Transmit (slow as ****)
+ *	
+ *		re-enable the IRQ
+ *	
+ *	
+ *	We have to use disable_irq because otherwise you will get delayed
+ *	interrupts on the APIC bus deadlocking the transmit path.
+ *	
+ *	Quite hairy but the chip simply wasn't designed for SMP and you can't
+ *	even ACK an interrupt without risking corrupting other parallel
+ *	activities on the chip." [lkml, 25 Jul 2007]
  */