Messages in this thread | | | Date | Sat, 14 Oct 2023 01:28:23 +0530 | Subject | Re: [PATCH 2/2] usb: gadget: ncm: Add support to update wMaxSegmentSize via configfs | From | Krishna Kurapati PSSNV <> |
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On 10/14/2023 12:09 AM, Maciej Żenczykowski wrote: > On Thu, Oct 12, 2023 at 8:40 AM Krishna Kurapati PSSNV > <quic_kriskura@quicinc.com> wrote: >> >> >> >> On 10/12/2023 6:02 PM, Maciej Żenczykowski wrote: >>> On Thu, Oct 12, 2023 at 1:48 AM Krishna Kurapati PSSNV >>> >>> Could you paste the full patch? >>> This is hard to review without looking at much more context then email >>> is providing >>> (or, even better, send me a link to a CL in gerrit somewhere - for >>> example aosp ACK mainline tree) >> >> Sure. Will provide a gerrit on ACK for review before posting v2. >> >> The intent of posting the diff was two fold: >> >> 1. The question Greg asked regarding why the max segment size was >> limited to 15014 was valid. When I thought about it, I actually wanted >> to limit the max MTU to 15000, so the max segment size automatically >> needs to be limited to 15014. > > Note that this is a *very* abstract value. > I get you want L3 MTU of 10 * 1500, but this value is not actually meaningful. > > IPv4/IPv6 fragmentation and IPv4/IPv6 TCP segmentation > do not result in a trivial multiplication of the standard 1500 byte > ethernet L3 MTU. > Indeed aggregating 2 1500 L3 mtu frames results in *different* sized > frames depending on which type of aggregation you do. > (and for tcp it even depends on the number and size of tcp options, > though it is often assumed that those take up 12 bytes, since that's the > normal for Linux-to-Linux tcp connections) > > For example if you aggregate N standard Linux ipv6/tcp L3 1500 mtu frames, > this means you have > N frames: ethernet (14) + ipv6 (40) + tcp (20) + tcp options (12) + > payload (1500-12-20-40=1500-72=1428) > post aggregation: > 1 frame: ethernet (14) + ipv6 (40) + tcp (20) + tcp options (12) + > payload (N*1428) > > so N * 1500 == N * (72 + 1428) --> 1 * (72 + N * 1428) > > That value of 72 is instead 52 for 'standard Linux ipv4/tcp), > it's 40/60 if there's no tcp options (which I think happens when > talking to windows) > it's different still with ipv4 fragmentation... and again different > with ipv6 fragmentation... > etc. > > ie. 15000 L3 mtu is exactly as meaningless as 14000 L3 mtu. > Either way you don't get full frames. > > As such I'd recommend going with whatever is the largest mtu that can > be meaningfully made to fit in 16K with all the NCM header overhead. > That's likely closer to 15500-16000 (though I have *not* checked). > >> But my commit text didn't mention this >> properly which was a mistake on my behalf. But when I looked at the >> code, limiting the max segment size 15014 would force the practical >> max_mtu to not cross 15000 although theoretical max_mtu was set to: >> (GETHER_MAX_MTU_SIZE - 15412) during registration of net device. >> >> So my assumption of limiting it to 15000 was wrong. It must be limited >> to 15412 as mentioned in u_ether.c This inturn means we must limit >> max_segment_size to: >> GETHER_MAX_ETH_FRAME_LEN (GETHER_MAX_MTU_SIZE + ETH_HLEN) >> as mentioned in u_ether.c. >> >> I wanted to confirm that setting MAX_DATAGRAM_SIZE to >> GETHER_MAX_ETH_FRAME_LEN was correct. >> >> 2. I am not actually able to test with MTU beyond 15000. When my host >> device is a linux machine, the cdc_ncm.c limits max_segment_size to: >> CDC_NCM_MAX_DATAGRAM_SIZE 8192 /* bytes */ > > In practice you get 50% of the benefits of infinitely large mtu by > going from 1500 to ~2980. > you get 75% of the benefits by going to ~6K > you get 87.5% of the benefits by going to ~12K > the benefits of going even higher are smaller and smaller... > > If the host side is limited to 8192, maybe we should match that here too?
Hi Maciej,
Thanks for the detailed explanation. I agree with you on setting device side also to 8192 instead of what max_mtu is present in u_ether or practical max segment size possible.
> > But the host side limitation of 8192 doesn't seem particularly sane either... > Maybe we should relax that instead? > I really didn't understand why it was set to 8192 in first place.
> (especially since for things like tcp zero copy you want an mtu which > is slighly more then N * 4096, > ie. around 4.5KB, 8.5KB, 12.5KB or something like that) >
I am not sure about host mode completely. If we want to increase though, just increasing the MAX_DATAGRAM_SIZE to some bigger value help ? (I don't know the entire code of cdc_ncm, so I might be wrong).
Regards, Krishna,
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