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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>
hifive-unleashed-5.1
Jarek Poplawski 2007-07-26 14:44:01 +02:00 committed by Jeff Garzik
parent 9351982b25
commit 55b7b629b7
1 changed files with 46 additions and 0 deletions
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46
drivers/net/lib8390.c
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@ -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]
*/