netdev - Re: [PATCH net-next 5/8] ice: register 1588 PTP clock device object for E810 devices

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Date:   Mon, 14 Jun 2021 09:43:17 -0700
From:   Jacob Keller <jacob.e.keller@...el.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Tony Nguyen <anthony.l.nguyen@...el.com>,
        Richard Cochran <richardcochran@...il.com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, sassmann@...hat.com,
        Tony Brelinski <tonyx.brelinski@...el.com>
Subject: Re: [PATCH net-next 5/8] ice: register 1588 PTP clock device object
 for E810 devices



On 6/11/2021 2:18 PM, Jakub Kicinski wrote:
> On Fri, 11 Jun 2021 09:19:57 -0700 Tony Nguyen wrote:
>> +static u64
>> +ice_ptp_read_src_clk_reg(struct ice_pf *pf, struct ptp_system_timestamp *sts)
>> +{
>> +	struct ice_hw *hw = &pf->hw;
>> +	u32 hi, lo, lo2;
>> +	u8 tmr_idx;
>> +
>> +	tmr_idx = ice_get_ptp_src_clock_index(hw);
>> +	/* Read the system timestamp pre PHC read */
>> +	if (sts)
>> +		ptp_read_system_prets(sts);
>> +
>> +	lo = rd32(hw, GLTSYN_TIME_L(tmr_idx));
>> +
>> +	/* Read the system timestamp post PHC read */
>> +	if (sts)
>> +		ptp_read_system_postts(sts);
>> +
>> +	hi = rd32(hw, GLTSYN_TIME_H(tmr_idx));
>> +	lo2 = rd32(hw, GLTSYN_TIME_L(tmr_idx));
>> +
>> +	if (lo2 < lo) {
>> +		/* if TIME_L rolled over read TIME_L again and update
>> +		 * system timestamps
>> +		 */
>> +		if (sts)
>> +			ptp_read_system_prets(sts);
>> +		lo = rd32(hw, GLTSYN_TIME_L(tmr_idx));
>> +		if (sts)
>> +			ptp_read_system_postts(sts);
> 
> ptp_read_system* helpers already check for NULL sts.
>

Hah. Yep, I knew that... and of course I forgot about it.

> 
>> +static int ice_ptp_adjfine(struct ptp_clock_info *info, long scaled_ppm)
>> +{
>> +	struct ice_pf *pf = ptp_info_to_pf(info);
>> +	u64 freq, divisor = 1000000ULL;
>> +	struct ice_hw *hw = &pf->hw;
>> +	s64 incval, diff;
>> +	int neg_adj = 0;
>> +	int err;
>> +
>> +	incval = ICE_PTP_NOMINAL_INCVAL_E810;
>> +
>> +	if (scaled_ppm < 0) {
>> +		neg_adj = 1;
>> +		scaled_ppm = -scaled_ppm;
>> +	}
>> +
>> +	while ((u64)scaled_ppm > div_u64(U64_MAX, incval)) {
>> +		/* handle overflow by scaling down the scaled_ppm and
>> +		 * the divisor, losing some precision
>> +		 */
>> +		scaled_ppm >>= 2;
>> +		divisor >>= 2;
>> +	}
> 
> I have a question regarding ppm overflows.
> 
> We have the max_adj field in struct ptp_clock_info which is checked
> against ppb, but ppb is a signed 32 bit and scaled_ppm is a long,
> meaning values larger than S32_MAX << 16 / 1000 will overflow 
> the ppb calculation, and therefore the check.
> 

Hmmm.. I thought ppb was a s64, not an s32.

In general, I believe max_adj is usually capped at 1 billion anyways,
since it doesn't make sense to slow a clock by more than 1billioln ppb,
and increasing it more than that isn't really useful either.

> Are we okay with that? Is my math off? Did I miss some part 
> of the kernel which filters crazy high scaled_ppm/freq?
> 
> Since dialed_freq is updated regardless of return value of .adjfine 
> the driver has no clear way to reject bad scaled_ppm>

I'm not sure. +Richard?

>> +	freq = (incval * (u64)scaled_ppm) >> 16;
>> +	diff = div_u64(freq, divisor);