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Add support for the Signal Quality Indicator (SQI) feature on KSZ9477 family switches, providing a relative measure of receive signal quality. The hardware exposes separate SQI readings per channel. For 1000BASE-T, all four channels are read. For 100BASE-TX, only one channel is reported, but which receive pair is active depends on Auto MDI-X negotiation, which is not exposed by the hardware. Therefore, it is not possible to reliably map the measured channel to a specific wire pair. This resolves an earlier discussion about how to handle multi-channel SQI. Originally, the plan was to expose all channels individually. However, since pair mapping is sometimes unavailable, this implementation treats SQI as a per-link metric instead. This fallback avoids ambiguity and ensures consistent behavior. The existing get_sqi() UAPI was designed for single-pair Ethernet (SPE), where per-pair and per-link are effectively equivalent. Restricting its use to per-link metrics does not introduce regressions for existing users. The raw 7-bit SQI value (0–127, lower is better) is converted to the standard 0–7 (high is better) scale. Empirical testing showed that the link becomes unstable around a raw value of 8. The SQI raw value remains zero if no data is received, even if noise is present. This confirms that the measurement reflects the "quality" during active data reception rather than the passive line state. User space must ensure that traffic is present on the link to obtain valid SQI readings. Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de> Link: https://patch.msgid.link/20250627112539.895255-1-o.rempel@pengutronix.de Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Linux kernel
============
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In order to build the documentation, use ``make htmldocs`` or
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There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
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the problems which may result by upgrading your kernel.