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[2a01:cb00:8ef9:5300:877b:aaab:2ea7:8695]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43970bfa015sm32337470f8f.8.2026.02.24.23.48.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 24 Feb 2026 23:48:09 -0800 (PST) Message-ID: <28cd2d03-6491-490d-89e2-19f81e9bad69@redhat.com> Date: Wed, 25 Feb 2026 08:48:08 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: commit 662dc80a5e86 breaks rmnet over usb To: Daniele Palmas Cc: Koen Vandeputte , oneukum@suse.com, andrew+netdev@lunn.ch, Eric Dumazet , kuba@kernel.org, pabeni@redhat.com, netdev , linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org References: <95ccf5a6-975b-4543-80a3-595f1ee20543@redhat.com> Content-Language: en-US From: Laurent Vivier Autocrypt: addr=lvivier@redhat.com; keydata= xsFNBFYFJhkBEAC2me7w2+RizYOKZM+vZCx69GTewOwqzHrrHSG07MUAxJ6AY29/+HYf6EY2 WoeuLWDmXE7A3oJoIsRecD6BXHTb0OYS20lS608anr3B0xn5g0BX7es9Mw+hV/pL+63EOCVm SUVTEQwbGQN62guOKnJJJfphbbv82glIC/Ei4Ky8BwZkUuXd7d5NFJKC9/GDrbWdj75cDNQx UZ9XXbXEKY9MHX83Uy7JFoiFDMOVHn55HnncflUncO0zDzY7CxFeQFwYRbsCXOUL9yBtqLer Ky8/yjBskIlNrp0uQSt9LMoMsdSjYLYhvk1StsNPg74+s4u0Q6z45+l8RAsgLw5OLtTa+ePM JyS7OIGNYxAX6eZk1+91a6tnqfyPcMbduxyBaYXn94HUG162BeuyBkbNoIDkB7pCByed1A7q q9/FbuTDwgVGVLYthYSfTtN0Y60OgNkWCMtFwKxRaXt1WFA5ceqinN/XkgA+vf2Ch72zBkJL RBIhfOPFv5f2Hkkj0MvsUXpOWaOjatiu0fpPo6Hw14UEpywke1zN4NKubApQOlNKZZC4hu6/ 8pv2t4HRi7s0K88jQYBRPObjrN5+owtI51xMaYzvPitHQ2053LmgsOdN9EKOqZeHAYG2SmRW LOxYWKX14YkZI5j/TXfKlTpwSMvXho+efN4kgFvFmP6WT+tPnwARAQABzSNMYXVyZW50IFZp dmllciA8bHZpdmllckByZWRoYXQuY29tPsLBeAQTAQIAIgUCVgVQgAIbAwYLCQgHAwIGFQgC CQoLBBYCAwECHgECF4AACgkQ8ww4vT8vvjwpgg//fSGy0Rs/t8cPFuzoY1cex4limJQfReLr SJXCANg9NOWy/bFK5wunj+h/RCFxIFhZcyXveurkBwYikDPUrBoBRoOJY/BHK0iZo7/WQkur 6H5losVZtrotmKOGnP/lJYZ3H6OWvXzdz8LL5hb3TvGOP68K8Bn8UsIaZJoeiKhaNR0sOJyI YYbgFQPWMHfVwHD/U+/gqRhD7apVysxv5by/pKDln1I5v0cRRH6hd8M8oXgKhF2+rAOL7gvh jEHSSWKUlMjC7YwwjSZmUkL+TQyE18e2XBk85X8Da3FznrLiHZFHQ/NzETYxRjnOzD7/kOVy gKD/o7asyWQVU65mh/ECrtjfhtCBSYmIIVkopoLaVJ/kEbVJQegT2P6NgERC/31kmTF69vn8 uQyW11Hk8tyubicByL3/XVBrq4jZdJW3cePNJbTNaT0d/bjMg5zCWHbMErUib2Nellnbg6bc 2HLDe0NLVPuRZhHUHM9hO/JNnHfvgiRQDh6loNOUnm9Iw2YiVgZNnT4soUehMZ7au8PwSl4I KYE4ulJ8RRiydN7fES3IZWmOPlyskp1QMQBD/w16o+lEtY6HSFEzsK3o0vuBRBVp2WKnssVH qeeV01ZHw0bvWKjxVNOksP98eJfWLfV9l9e7s6TaAeySKRRubtJ+21PRuYAxKsaueBfUE7ZT 7zfOwU0EVgUmGQEQALxSQRbl/QOnmssVDxWhHM5TGxl7oLNJms2zmBpcmlrIsn8nNz0rRyxT 460k2niaTwowSRK8KWVDeAW6ZAaWiYjLlTunoKwvF8vP3JyWpBz0diTxL5o+xpvy/Q6YU3BN efdq8Vy3rFsxgW7mMSrI/CxJ667y8ot5DVugeS2NyHfmZlPGE0Nsy7hlebS4liisXOrN3jFz asKyUws3VXek4V65lHwB23BVzsnFMn/bw/rPliqXGcwl8CoJu8dSyrCcd1Ibs0/Inq9S9+t0 VmWiQWfQkz4rvEeTQkp/VfgZ6z98JRW7S6l6eophoWs0/ZyRfOm+QVSqRfFZdxdP2PlGeIFM C3fXJgygXJkFPyWkVElr76JTbtSHsGWbt6xUlYHKXWo+xf9WgtLeby3cfSkEchACrxDrQpj+ Jt/JFP+q997dybkyZ5IoHWuPkn7uZGBrKIHmBunTco1+cKSuRiSCYpBIXZMHCzPgVDjk4viP brV9NwRkmaOxVvye0vctJeWvJ6KA7NoAURplIGCqkCRwg0MmLrfoZnK/gRqVJ/f6adhU1oo6 z4p2/z3PemA0C0ANatgHgBb90cd16AUxpdEQmOCmdNnNJF/3Zt3inzF+NFzHoM5Vwq6rc1JP jfC3oqRLJzqAEHBDjQFlqNR3IFCIAo4SYQRBdAHBCzkM4rWyRhuVABEBAAHCwV8EGAECAAkF AlYFJhkCGwwACgkQ8ww4vT8vvjwg9w//VQrcnVg3TsjEybxDEUBm8dBmnKqcnTBFmxN5FFtI WlEuY8+YMiWRykd8Ln9RJ/98/ghABHz9TN8TRo2b6WimV64FmlVn17Ri6FgFU3xNt9TTEChq AcNg88eYryKsYpFwegGpwUlaUaaGh1m9OrTzcQy+klVfZWaVJ9Nw0keoGRGb8j4XjVpL8+2x OhXKrM1fzzb8JtAuSbuzZSQPDwQEI5CKKxp7zf76J21YeRrEW4WDznPyVcDTa+tz++q2S/Bp P4W98bXCBIuQgs2m+OflERv5c3Ojldp04/S4NEjXEYRWdiCxN7ca5iPml5gLtuvhJMSy36gl U6IW9kn30IWuSoBpTkgV7rLUEhh9Ms82VWW/h2TxL8enfx40PrfbDtWwqRID3WY8jLrjKfTd R3LW8BnUDNkG+c4FzvvGUs8AvuqxxyHbXAfDx9o/jXfPHVRmJVhSmd+hC3mcQ+4iX5bBPBPM oDqSoLt5w9GoQQ6gDVP2ZjTWqwSRMLzNr37rJjZ1pt0DCMMTbiYIUcrhX8eveCJtY7NGWNyx FCRkhxRuGcpwPmRVDwOl39MB3iTsRighiMnijkbLXiKoJ5CDVvX5yicNqYJPKh5MFXN1bvsB kmYiStMRbrD0HoY1kx5/VozBtc70OU0EB8Wrv9hZD+Ofp0T3KOr1RUHvCZoLURfFhSQ= In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2/25/26 08:19, Daniele Palmas wrote: > Hello, Hello Daniele, > > Il giorno lun 23 feb 2026 alle ore 15:08 Laurent Vivier > ha scritto: >> >> On 2/23/26 13:04, Koen Vandeputte wrote: >>> Hi Laurent, >> >> Hi Koen, >> >>> I'm testing the latest openwrt state and found an issue probably >>> caused by your usb mtu limit patch :-) >>> >>> I'm talking about this one: >>> 662dc80a5e86 ("usbnet: limit max_mtu based on device's hard_mtu") >>> >>> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=v6.12.74&id=662dc80a5e86b35bbf339e0b87b7cc3f07c09de1 >>> >>> >>> When using wwan0 iface normally, this makes sense, but the problem is >>> when using QMI modems combined with the rmnet driver and aggregated >>> frames. >>> >>> - The modem is configured to frame sizes of 16383 or 32767 using QMI >>> - wwan0 (using qmi_wwan) is configured to match this frame size by >>> setting it's MTU to the same value >>> - Frames of this size are sent over to qmi_wwan driver (containing >>> multiple data packets) >>> - Frames are then forwarded to the rmnet driver >>> - Frames get de-aggregated here and sent to the network stack for processing. >>> >>> The reason here is to reduce USB transfers heavily. >>> >>> >>> As you see, it's perfectly possible here to use very large MTU sizes >>> as the aggregation feature by rmnet relies on this. >>> Also the modem can be perfectly configured to send very large aggregated frames. >>> >>> After your patch, wwan0 is limited to 1500 bytes it seems, effectively >>> breaking aggregation. >>> >>> On my tests, download speeds are reduced from >300Mbps to ~.8Mbps >>> >>> I also made a build reverting this patch and all works well again. >>> >>> >>> Is there any other solution to fix this? >>> I guess it should be reverted otherwise :-) >> >> It's weird to be able to set an MTU that is not supported by the hardware. >> >> To restore performance I think the rx_usb_size should be decoupled from MTU max in qmi_wwan. >> >> Could you try something like: >> >> diff --git a/drivers/net/usb/qmi_wwan.c b/drivers/net/usb/qmi_wwan.c >> index 3a4985b582cb..6b4796fac692 100644 >> --- a/drivers/net/usb/qmi_wwan.c >> +++ b/drivers/net/usb/qmi_wwan.c >> @@ -788,6 +788,8 @@ static int qmi_wwan_bind(struct usbnet *dev, struct usb_interface *intf) >> usbnet_get_ethernet_addr(dev, cdc_ether->iMACAddress); >> } >> >> + dev->rx_urb_size = 32768; >> + > > So far userspace tools (e.g. also the most important one which is > ModemManager) rely on changing the rx_urb_size by changing the MTU: I > know this is ugly, but it is a behavior that has been there since a > lot of time, not sure how many tools based on this assumption could > break. > > There's also the chance that there are modems which require a higher > rx_urb_size, so having this fixed could not work well. > > Unfortunately usbnet serves many drivers, I agree with Koen that a > revert is the safest option. And there is no intermediate driver (qmi_wwan or rmnet) that can define a max_mtu higher than that defined by usbnet? Thanks, Laurent