Russell,

On Fri, 6 Jan 2017 12:59:24 +0000, Russell King - ARM Linux wrote:

Beware of rounding and overflow errors.  usec and val are u32's.

[...]

Thanks for all the suggestions, I've taken this into account in my v3
that I've sent a few minutes ago? I've re-used almost exactly your
implementation, with a few changes (see below).

static u32 usec_to_cycles(u32 usec, unsigned long clock_rate_hz)
{
	u64 tmp = clock_rate_hz * usec;

I had to cast one of the variables to u64 here otherwise the
multiplication was done on 32 bits, and then the result was expanded to
64 bits.

	do_div(tmp, USEC_PER_SEC);

	return tmp > 0xffffffff ? 0xffffffff : tmp;

I've used U32_MAX here.

static u32 cycles_to_usec(u32 cycles, unsigned long clock_rate_hz)
{
	u64 tmp = cycles * USEC_PER_SEC;

	do_div(tmp, clock_rate_hz);

	return tmp > 0xffffffff ? 0xffffffff : tmp;

Same comments for this function.

and:
	u32 val = usec_to_cycles(usec, port->priv->tclk);

	if (val > MVPP2_MAX_ISR_RX_THRESHOLD) {
		usec = cycles_to_usec(MVPP2_MAX_ISR_RX_THRESHOLD,
				      port->priv->tclk);

		/* re-evaluate to get actual register value for usec */
		val = usec_to_cycles(usec, port->priv->tclk);
	}

quoted

 	mvpp2_write(port->priv, MVPP2_ISR_RX_THRESHOLD_REG(rxq->id), val);
 
 	rxq->time_coal = usec;  

This function appears to be called from two places:

mvpp2_rxq_init():
        mvpp2_rx_time_coal_set(port, rxq, rxq->time_coal);

mvpp2_ethtool_set_coalesce():
                rxq->time_coal = c->rx_coalesce_usecs;
                mvpp2_rx_time_coal_set(port, rxq, rxq->time_coal);

It seems rather pointless to pass in rxq->time_coal when you're also
passing in rxq.

Indeed, so I've added another patch in the series that does this.

Thanks for the review!

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com