Subject: Re: [net-next, v3, 01/10] ptp: add ptp virtual clock driver framework

From: Richard Cochran <[email-masked]>
Date: Thu, 17 Jun 2021 19:32:38 +0200

On Tue, Jun 15, 2021 at 05:45:08PM +0800, Yangbo Lu wrote:
> diff --git a/drivers/ptp/Makefile b/drivers/ptp/Makefile
> index 8673d1743faa..3c6a905760e2 100644
> --- a/drivers/ptp/Makefile
> +++ b/drivers/ptp/Makefile
> @@ -3,7 +3,7 @@
>  # Makefile for PTP 1588 clock support.
>  #
>  
> -ptp-y					:= ptp_clock.o ptp_chardev.o ptp_sysfs.o
> +ptp-y					:= ptp_clock.o ptp_vclock.o ptp_chardev.o ptp_sysfs.o

Nit: Please place ptp_vclock.o at the end of the list.

>  ptp_kvm-$(CONFIG_X86)			:= ptp_kvm_x86.o ptp_kvm_common.o
>  ptp_kvm-$(CONFIG_HAVE_ARM_SMCCC)	:= ptp_kvm_arm.o ptp_kvm_common.o
>  obj-$(CONFIG_PTP_1588_CLOCK)		+= ptp.o

> diff --git a/drivers/ptp/ptp_private.h b/drivers/ptp/ptp_private.h
> index 6b97155148f1..3f388d63904c 100644
> --- a/drivers/ptp/ptp_private.h
> +++ b/drivers/ptp/ptp_private.h
> @@ -48,6 +48,20 @@ struct ptp_clock {
>  	struct kthread_delayed_work aux_work;
>  };
>  
> +#define info_to_vclock(d) container_of((d), struct ptp_vclock, info)
> +#define cc_to_vclock(d) container_of((d), struct ptp_vclock, cc)
> +#define dw_to_vclock(d) container_of((d), struct ptp_vclock, refresh_work)
> +
> +struct ptp_vclock {
> +	struct ptp_clock *pclock;
> +	struct ptp_clock_info info;
> +	struct ptp_clock *clock;
> +	struct cyclecounter cc;
> +	struct timecounter tc;
> +	spinlock_t lock;	/* protects tc/cc */
> +	struct delayed_work refresh_work;

Can we please have a kthread worker here instead of work?

Experience shows that plain work can be delayed for a long, long time
on busy systems.

> +};
> +
>  /*
>   * The function queue_cnt() is safe for readers to call without
>   * holding q->lock. Readers use this function to verify that the queue
> @@ -89,4 +103,6 @@ extern const struct attribute_group *ptp_groups[];
>  int ptp_populate_pin_groups(struct ptp_clock *ptp);
>  void ptp_cleanup_pin_groups(struct ptp_clock *ptp);
>  
> +struct ptp_vclock *ptp_vclock_register(struct ptp_clock *pclock);
> +void ptp_vclock_unregister(struct ptp_vclock *vclock);
>  #endif

> diff --git a/drivers/ptp/ptp_vclock.c b/drivers/ptp/ptp_vclock.c
> new file mode 100644
> index 000000000000..b8f500677314
> --- /dev/null
> +++ b/drivers/ptp/ptp_vclock.c
> @@ -0,0 +1,154 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * PTP virtual clock driver
> + *
> + * Copyright 2021 NXP
> + */
> +#include <linux/slab.h>
> +#include "ptp_private.h"
> +
> +#define PTP_VCLOCK_CC_MULT		(1 << 31)
> +#define PTP_VCLOCK_CC_SHIFT		31

These two are okay, but ...

> +#define PTP_VCLOCK_CC_MULT_NUM		(1 << 9)
> +#define PTP_VCLOCK_CC_MULT_DEM		15625ULL

the similarity of naming is confusing for these two.  They are only
used in the .adjfine method.  How about this?

  PTP_VCLOCK_FADJ_NUMERATOR, or even PTP_VCLOCK_FADJ_SHIFT (see below)
  PTP_VCLOCK_FADJ_DENOMINATOR

> +#define PTP_VCLOCK_CC_REFRESH_INTERVAL	(HZ * 2)

Consider dropping CC from the name.
PTP_VCLOCK_REFRESH_INTERVAL sounds good to me.

> +static int ptp_vclock_adjfine(struct ptp_clock_info *ptp, long scaled_ppm)
> +{
> +	struct ptp_vclock *vclock = info_to_vclock(ptp);
> +	unsigned long flags;
> +	s64 adj;
> +
> +	adj = (s64)scaled_ppm * PTP_VCLOCK_CC_MULT_NUM;

Rather than

    scaled_ppm * (1 << 9)

I suggest

    scaled_ppm << 9

instead.  I suppose a good compiler would replace the multiplication
with a bit shift, but it never hurts to spell it out.

> +	adj = div_s64(adj, PTP_VCLOCK_CC_MULT_DEM);
> +
> +	spin_lock_irqsave(&vclock->lock, flags);
> +	timecounter_read(&vclock->tc);
> +	vclock->cc.mult = PTP_VCLOCK_CC_MULT + adj;
> +	spin_unlock_irqrestore(&vclock->lock, flags);
> +
> +	return 0;
> +}

Thanks,
Richard