netdev - Re: [PATCH net-next v2 1/2] bnxt_en: cache only 24 bits of hw counter

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Message-ID: <CACKFLinHAXcXF45NTJueBg8JDbJfPTrPZiwHzR71K4LtvHxLVw@mail.gmail.com>
Date: Fri, 25 Oct 2024 15:13:33 -0700
From: Michael Chan <michael.chan@...adcom.com>
To: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
Cc: Pavan Chebbi <pavan.chebbi@...adcom.com>, Jakub Kicinski <kuba@...nel.org>, 
	Andrew Lunn <andrew+netdev@...n.ch>, Paolo Abeni <pabeni@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org, 
	Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH net-next v2 1/2] bnxt_en: cache only 24 bits of hw counter

On Fri, Oct 25, 2024 at 2:53 PM Vadim Fedorenko
<vadim.fedorenko@...ux.dev> wrote:
>
> On 25/10/2024 22:31, Michael Chan wrote:
> > On Fri, Oct 25, 2024 at 12:48 PM Vadim Fedorenko <vadfed@...a.com> wrote:
> >>
> >> This hardware can provide only 48 bits of cycle counter. We can leave
> >> only 24 bits in the cache to extend RX timestamps from 32 bits to 48
> >> bits. This make cache writes atomic even on 32 bit platforms and we can
> >> simply use READ_ONCE()/WRITE_ONCE() pair and remove spinlock. The
> >> configuration structure will be also reduced by 4 bytes.
> >
> > ptp->old_time serves 2 purposes: to cache the upper 16 bits of the HW
> > counter and for rollover check.  With this patch reducing
> > ptp->old_time to 24 bits, we now use the upper 16 bits for the cache
> > and the next 8 bits for the rollover check.  I think this will work.
> > But since the field is now 32-bit, why not use the full 32 bits
> > instead of 24 bits?  Thanks.
>
> As you confirmed that the HW has 48 bits of cycle counter, we have to
> cache 16 bits only. The other 8 bits are used for the rollover check. We
> can even use less bits for it. What is the use for another 8 bits? With
> this patch the rollover will happen every ~16 ms, but will be caught
> even till ~4s. If we will cache upper 32 bits, the rollover will happen
> every 64us and we will have to update cache value more often. But at the
> same time it will not bring any value, because the upper limit will
> still be ~4s. That's why I don't see any benefits of using all 32 bits.

I agree the extra bits have no value other than to fill the 32-bit
field.  But it should not affect the frequency of caching
ptp->old_time.  It should be updated in bnxt_ptp_ts_aux_work() at a
fixed interval (1 second).

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