Messages in this thread | | | Date | Mon, 05 May 2014 17:09:59 -0400 (EDT) | Subject | Re: [PATCH 5/5] net: macb: Fix race between HW and driver | From | David Miller <> |
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From: Sören Brinkmann <soren.brinkmann@xilinx.com> Date: Mon, 5 May 2014 14:05:19 -0700
> On Mon, 2014-05-05 at 04:56PM -0400, David Miller wrote: >> From: Soren Brinkmann <soren.brinkmann@xilinx.com> >> Date: Sun, 4 May 2014 15:43:02 -0700 >> >> > 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 <soren.brinkmann@xilinx.com> >> >> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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