net_test_phy...tcp_bad_csum(9)

💅 NAME 💅

okay so bestie, gather round — this is net_test_phy_loopback_tcp_bad_csum, and it's literally the PHY loopback test that ships a deliberately corrupted TCP checksum. yes, on purpose. we're testing the hardware's vibe check, not its math. 🎀

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✨ SYNOPSIS ✨

int net_test_phy_loopback_tcp_bad_csum (struct net_device *ndev );

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🦋 ARGUMENTS 🦋

• ndev — the network device you're putting through the test. the main character of this whole operation, honestly. 👑

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🌸 DESCRIPTION 🌸

So here's the tea: this builds the exact same minimal Ethernet/IPv4/TCP frame as net_test_phy_loopback_tcp, and then it flips the least-significant bit of the TCP checksum 💀. which means the resulting value is provably invalid — it's neither 0 nor 0xFFFF. no cap, it cannot sneak through as valid.

Then that frame gets transmitted through the device's internal PHY loopback path, and the journey looks like this:

test code -> MAC driver -> MAC HW -> xMII -> PHY -> internal PHY
loopback -> xMII -> MAC HW -> MAC driver -> test code

it's giving full circle. iconic. 🔥

👀 Result interpretation 👀

• 0 — the frame is delivered to the stack, and the driver reports ip_summed as CHECKSUM_NONE or CHECKSUM_COMPLETE. both are valid ways to say "bad checksum, let the stack verify." slay, that's the correct behavior. ✅
• -ETIMEDOUT — the MAC/PHY silently dropped the frame. hardware checksum verification filtered it out before the driver ever saw it. 🫥
• -EIO — the driver returned the frame with ip_summed == CHECKSUM_UNNECESSARY, which is it falsely claiming a valid checksum. that indicates a serious RX-path defect. not cute. not the vibe. red flag fr fr. 🚩

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💖 RETURN 💖

0 on success, or a negative error code on failure. simple as that, bestie. 🫶

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🌈 SEE ALSO 🌈

• Kernel file ./net/core/selftests.c

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and that's the whole page, bestie — hope you understood it, iykyk 💜 this little gem lives under ./net/core, dated October 2026, and the manpage is all about net_test_phy...tcp_bad_csum(9) in the Kernel API Manual. go forth and test those checksums 🧚✨