Messages in this thread | | | Date | Fri, 20 Aug 2021 19:41:00 +0100 | From | Marc Zyngier <> | Subject | Re: [PATCH net-next] stmmac: align RX buffers |
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On Fri, 20 Aug 2021 19:14:22 +0100, Matteo Croce <mcroce@linux.microsoft.com> wrote: > > On Fri, Aug 20, 2021 at 8:09 PM Marc Zyngier <maz@kernel.org> wrote: > > > > On Fri, 20 Aug 2021 18:56:33 +0100, > > Matteo Croce <mcroce@linux.microsoft.com> wrote: > > > > > > On Fri, Aug 20, 2021 at 7:51 PM Marc Zyngier <maz@kernel.org> wrote: > > > > > > > > On Fri, 20 Aug 2021 18:35:45 +0100, > > > > Matteo Croce <mcroce@linux.microsoft.com> wrote: > > > > > > > > > > > > I think it's wrong. The original offset was 0, and to align it to the > > > > > > > boundary we need to add just NET_IP_ALIGN, which is two. > > > > > > > NET_SKB_PAD is a much bigger value, (I think 64), which is used to > > > > > > > reserve space to prepend an header, e.g. with tunnels. > > > > > > > > > > > > How about the other adjustments that Eric mentioned regarding the size > > > > > > of the buffer? Aren't they required? > > > > > > > > > > > > > > > > I guess that if stmmac_rx_buf1_len() needed such adjustment, it would > > > > > be already broken when XDP is in use. > > > > > When you use XDP, stmmac_rx_offset() adds a pretty big headroom of 256 > > > > > byte, which would easily trigger an overflow if not accounted. > > > > > Did you try attaching a simple XDP program on a stock 5.13 kernel? > > > > > > > > Yes, as mentioned in [1], to which you replied... > > > > > > > > M. > > > > > > > > [1] https://lore.kernel.org/r/87wnohqty1.wl-maz@kernel.org > > > > > > > > > > Great. > > > So I doubt that the adjustment is needed. > > > Does it work with all the frame size? > > > > I have no idea. Honestly, you are the one who should be able to answer > > these questions, given that you should have worked out how the buffer > > allocations work in this particular driver. > > > > This whole "let's try another random set of values until something > > sticks" is not how things ought to be done, and doesn't fill me with > > the utmost confidence that 5.14 (which apparently may well be cut in > > *two days*) is going to have a solid stmmac driver. > > > > I re-re-request that this patch gets reverted until you figure out > > what is wrong with the initial patch. > > > > Thanks, > > > > I would have done it, but I'll not have the hardware until next week at least, > otherwise I'd have tried all these tests myself. > > I'm sure that NET_SKB_PAD doesn't need to be there, if just removing > it fixes the problem, consider applying it and put a Fixes tag.
No, I don't think that's the right thing to do. A patch breaks a driver, and the author of the patch is not in a position to fix it. That's OK, these things happen, it's just bad timing.
But I don't understand this part of the kernel well enough to submit a patch based on a sample of *one*, at the last minute, just because "it works for me", and have the confidence that it doesn't break anything else.
I have now posted a revert of the original patch[1]. I'll be happy to work with you, with a less pressure, in order to have something that works for everyone in the next cycle.
Thanks,
M.
[1] https://lore.kernel.org/netdev/20210820183002.457226-1-maz@kernel.org
-- Without deviation from the norm, progress is not possible.
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