Re: [PATCH net-next v3 2/3] ptp: Add driver for R-Car Gen4

[Vadim Fedorenko <vadim.fedorenko@linux.dev>]

On 01/07/2026 10:06, Niklas Söderlund wrote:
> Add driver for the gPTP timer found on R-Car Gen4 devices. The timer is
> system-wide and shared by different Ethernet devices on each Gen4
> platform. The operation of the timer is however not completely in
> depended of the systems Ethernet devices.
> 
>    - On R-Car S4 is gated by the RSWITCH Ethernet module clock.
> 
>    - On R-Car V4H is gated by the RTSN Ethernet module clock.
> 
>    - On R-Car V4M is gated by its own module clock, the system have
>      neither RTSN or RSWITCH device. But the module clock is the same as
>      RTSN on V4H and the documentation referees to it as tsn (EtherTSN).
> 
> The gPTP device do have its own register space on all three platforms.
> But on S4 and V4H it will share its clock and reset property with
> RSWITCH or RTSN, respectively.
> 
> Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>

[...]

> +static int ptp_rcar_gen4_adjfine(struct ptp_clock_info *ptp, long scaled_ppm)
> +{
> +	struct ptp_rcar_gen4_priv *priv = ptp_to_priv(ptp);
> +	s64 addend = priv->default_addend;
> +	bool neg_adj = scaled_ppm < 0;
> +	unsigned long flags;
> +	s64 diff;
> +
> +	if (neg_adj)
> +		scaled_ppm = -scaled_ppm;
> +	diff = div_s64(addend * scaled_ppm_to_ppb(scaled_ppm), NSEC_PER_SEC);
> +	addend = neg_adj ? addend - diff : addend + diff;
> +
> +	spin_lock_irqsave(&priv->lock, flags);
> +	iowrite32(addend, priv->base + PTPTIVC0_REG);

how are you so sure that addend will always fit into s32? It looks like
it may go over in some cases, no?

> +	spin_unlock_irqrestore(&priv->lock, flags);
> +
> +	return 0;
> +}