Re: [PATCH net-next v2 2/2] net: stmmac: Add a devlink attribute to control timestamping mode

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Fri, Oct 24, 2025 at 09:07:18AM +0200, Maxime Chevallier wrote:
> The DWMAC1000 supports 2 timestamping configurations to configure how
> frequency adjustments are made to the ptp_clock, as well as the reported
> timestamp values.
> 
> There was a previous attempt at upstreaming support for configuring this
> mode by Olivier Dautricourt and Julien Beraud a few years back [1]
> 
> In a nutshell, the timestamping can be either set in fine mode or in
> coarse mode.
> 
> In fine mode, which is the default, we use the overflow of an accumulator to
> trigger frequency adjustments, but by doing so we lose precision on the
> timetamps that are produced by the timestamping unit. The main drawback
> is that the sub-second increment value, used to generate timestamps, can't be
> set to lower than (2 / ptp_clock_freq).
> 
> The "fine" qualification comes from the frequent frequency adjustments we are
> able to do, which is perfect for a PTP follower usecase.
> 
> In Coarse mode, we don't do frequency adjustments based on an
> accumulator overflow. We can therefore have very fine subsecond
> increment values, allowing for better timestamping precision. However
> this mode works best when the ptp clock frequency is adjusted based on
> an external signal, such as a PPS input produced by a GPS clock. This
> mode is therefore perfect for a Grand-master usecase.
> 
> Introduce a driver-specific devlink parameter "ts_coarse" to enable or
> disable coarse mode, keeping the "fine" mode as a default.
> 
> This can then be changed with:
> 
>   devlink dev param set <dev> name ts_coarse value true cmode runtime
> 
> The associated documentation is also added.
> 
> [1] : https://lore.kernel.org/netdev/20200514102808.31163-1-olivier.dautricourt@orolia.com/
> 
> Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

Thanks!

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