linux-kernel - Re: [PATCH net-next 09/16] net: mdio: Add Synopsys DW XPCS management interface support

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Message-ID: <19ebc244-6d79-4e12-8ac4-fcf046106a07@lunn.ch>
Date:   Thu, 7 Dec 2023 15:54:08 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Serge Semin <fancer.lancer@...il.com>
Cc:     Maxime Chevallier <maxime.chevallier@...tlin.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        Jose Abreu <Jose.Abreu@...opsys.com>,
        Tomer Maimon <tmaimon77@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, openbmc@...ts.ozlabs.org,
        netdev@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 09/16] net: mdio: Add Synopsys DW XPCS
 management interface support

On Thu, Dec 07, 2023 at 04:35:47PM +0300, Serge Semin wrote:
> Hi Andrew
> 
> On Wed, Dec 06, 2023 at 06:01:30PM +0100, Andrew Lunn wrote:
> > > > You shouldn't use inline in C files, only in headers.
> > > 
> > > Could you please clarify why? I failed to find such requirement in the
> > > coding style doc. Moreover there are multiple examples of using the
> > > static-inline-ers in the C files in the kernel including the net/mdio
> > > subsystem.
> > 
> 
> > The compiler does a better job at deciding what to inline than we
> > humans do. If you can show the compiler is doing it wrong, then we
> > might accept them.
> 
> In general I would have agreed with you especially if the methods were
> heavier than what they are:
> static inline ptrdiff_t dw_xpcs_mmio_addr_format(int dev, int reg)
> {               
>         return FIELD_PREP(0x1f0000, dev) | FIELD_PREP(0xffff, reg);
> }               
>         
> static inline u16 dw_xpcs_mmio_addr_page(ptrdiff_t csr)
> {       
>         return FIELD_GET(0x1fff00, csr);
> }       
> 
> static inline ptrdiff_t dw_xpcs_mmio_addr_offset(ptrdiff_t csr)
> {
>         return FIELD_GET(0xff, csr);
> }
> 
> > But in general, netdev does not like inline in .C
> > file.
> 
> I see. I'll do as you say if you don't change your mind after my
> reasoning below.
> 
> > Also, nothing in MDIO is hot path, it spends a lot of time
> > waiting for a slow bus. So inline is likely to just bloat the code for
> > no gain.
> 
> I would have been absolutely with you in this matter, if we were talking
> about a normal MDIO bus. In this case the devices are accessed over
> the system IO-memory. So the bus isn't that slow.

O.K, so its not slow. But how often is it used? PHYs tend to get
polled once a second if interrupts are not used. But is the PCS also
polled at the same time? Does this optimisation make a noticeable
difference at once per second? Do you have a requirement that the
system boots very very fast, and this optimisation makes a difference
when there is heavier activity on the PCS at boot? Is the saving
noticeable, when auto-neg takes a little over 1 second.

The best way to make your case is show real world requirements and
benchmark results.

	  Andrew