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>
diff --git a/drivers/net/lib8390.c b/drivers/net/lib8390.c
index 721ee38..c429a50 100644
--- a/drivers/net/lib8390.c
+++ b/drivers/net/lib8390.c
@@ -143,6 +143,52 @@
* 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]
*/