Re: [PATCH 5/5] net: macb: Fix race between HW and driver

From: Sören Brinkmann <redacted>
Date: Mon, 5 May 2014 14:05:19 -0700

On Mon, 2014-05-05 at 04:56PM -0400, David Miller wrote:

quoted

From: Soren Brinkmann <redacted>
Date: Sun,  4 May 2014 15:43:02 -0700

quoted

Under "heavy" RX load, the driver cannot handle the descriptors fast
enough. In detail, when a descriptor is consumed, its used flag is
cleared and once the RX budget is consumed all descriptors with a
cleared used flag are prepared to receive more data. Under load though,
the HW may constantly receive more data and use those descriptors with a
cleared used flag before they are actually prepared for next usage.

The head and tail pointers into the RX-ring should always be valid and
we can omit clearing and checking of the used flag.

Signed-off-by: Soren Brinkmann <redacted>

Isn't the RX_USED bit the only thing that controls what RX entries
the chip will try to use?

I can't see how you can just remove the RX_USED bit handling
altogether.

The problem actually seems to be that in the current code we clear the
RX_USED bit before we actually reallocate the buffer and set it up.

It should be a bug to see the RX_USED bit set in gem_rx_refill(), and
the only reason why it can happen is exactly because you're clearing it
too early in gem_rx().

I don't follow. The HW uses the descriptor and the driver handles the
received data. So, in gem_rx_refill we should actually only replace
descriptor which have the RX_USED _set_, not? Currently the test tests
for the opposite, since SW clears RX_USED in gem_rx. This patch just
removes those two parts. The RX_USED is left as is (HW should have set
it). And in gem_rx_refill we simply rely on the head and tail pointers
to refill the used descriptors. I didn't see a reason to do the additional
checking of the RX_USED bit.
After the refill the RX_USED flags are of course cleared for all
refilled descriptors.

Aha, that makes sense, I didn't notice that RX_USED is cleared as a side
effect of gem_rx_refill()'s work.

I'll apply this series, thanks for explaining.