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Date: Mon, 5 May 2014 14:10:23 -0700 From: Sören Brinkmann <soren.brinkmann@...inx.com> To: David Miller <davem@...emloft.net> CC: <michal.simek@...inx.com>, <nicolas.ferre@...el.com>, <git@...inx.com>, <linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>, <netdev@...r.kernel.org> Subject: Re: [PATCH 5/5] net: macb: Fix race between HW and driver On Mon, 2014-05-05 at 05:09PM -0400, David Miller wrote: > From: Sören Brinkmann <soren.brinkmann@...inx.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@...inx.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@...inx.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. Thanks, but we probably wanna wait for Nicolas to test on his platforms using macb? Sören -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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