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The IPQ4019 MDIO internally divide the clock feed by AHB based on the MDIO_MODE reg. On reset or power up, the default value for the divider is 0xff that reflect the divider set to /256. This makes the MDC run at a very low rate, that is, considering AHB is always fixed to 100Mhz, a value of 390KHz. This hasn't have been a problem as MDIO wasn't used for time sensitive operation, it is now that on IPQ807x is usually mounted with PHY that requires MDIO to load their firmware (example Aquantia PHY). To handle this problem and permit to set the correct designed MDC frequency for the SoC add support for the standard "clock-frequency" property for the MDIO node. The divider supports value from /1 to /256 and the common value are to set it to /16 to reflect 6.25Mhz or to /8 on newer platform to reflect 12.5Mhz. To scan if the requested rate is supported by the divider, loop with each supported divider and stop when the requested rate match the final rate with the current divider. An error is returned if the rate doesn't match any value. On MDIO reset, the divider is restored to the requested value to prevent any kind of downclocking caused by the divider reverting to a default value. To follow 802.3 spec of 2.5MHz of default value, if divider is set at /256 and "clock-frequency" is not set in DT, assume nobody set the divider and try to find the closest MDC rate to 2.5MHz. (in the case of AHB set to 100MHz, it's 1.5625MHz) While at is also document other bits of the MDIO_MODE reg to have a clear idea of what is actually applied there. Documentation of some BITs is skipped as they are marked as reserved and their usage is not clear (RES 11:9 GENPHY 16:13 RES1 19:17) Signed-off-by: Christian Marangi <ansuelsmth@gmail.com> Reviewed-by: Andrew Lunn <andrew@lunn.ch> Signed-off-by: David S. Miller <davem@davemloft.net> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.