Re: [PATCH 2/2] usb: gadget: ncm: Add support to update wMaxSegmentSize via configfs

[Krishna Kurapati PSSNV <quic_kriskura@quicinc.com>]

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,