Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 55b7b629 authored by Jarek Poplawski's avatar Jarek Poplawski Committed by Jeff Garzik
Browse files

lib8390: comment on locking by Alan Cox 



Additional explanation of problems with locking by Alan Cox.

Signed-off-by: default avatarJarek 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: default avatarJeff Garzik <jeff@garzik.org>
parent 9351982b

drivers/net/lib8390.c

+46 −0
Original line number Diff line number Diff line
@@ -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]

 */