linux-kernel - Re: [PATCH v3 01/10] of_net: add NVMEM support to of_get_mac

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Date:   Fri, 3 May 2019 11:15:42 +0200
From:   Petr Štetiar <ynezz@...e.cz>
To:     Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
Cc:     netdev@...r.kernel.org, devicetree@...r.kernel.org,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Alban Bedel <albeu@...e.fr>, Felix Fietkau <nbd@....name>,
        John Crispin <john@...ozen.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 01/10] of_net: add NVMEM support to of_get_mac_address

Sergei Shtylyov <sergei.shtylyov@...entembedded.com> [2019-05-03 11:44:54]:

Hi Sergei,

> > diff --git a/drivers/of/of_net.c b/drivers/of/of_net.c
> > index d820f3e..258ceb8 100644
> > --- a/drivers/of/of_net.c
> > +++ b/drivers/of/of_net.c
> [...]
> > @@ -64,6 +113,9 @@ static const void *of_get_mac_addr(struct device_node *np, const char *name)
> >    * addresses.  Some older U-Boots only initialized 'local-mac-address'.  In
> >    * this case, the real MAC is in 'local-mac-address', and 'mac-address' exists
> >    * but is all zeros.
> > + *
> > + * Return: Will be a valid pointer on success, NULL in case there wasn't
> > + *         'mac-address' nvmem cell node found, and ERR_PTR in case of error.
> 
>    Returning both NULL and error codes on failure is usually a sign of a
> misdesigned API. 

well, then there's a lot of misdesigned APIs in the tree already, as I've just
grepped for IS_ERR_OR_NULL usage and found this pointer/NULL/ERR_PTR usage
pretty legit.

> Why not always return an error code?

I've received following comment[1] from Andrew:

 "What you have to be careful of, is the return value from your new code
  looking in NVMEM. It should only return EPROBE_DEFER, or another error
  if there really is expected to be a value in NVMEM, or getting it from
  NVMEM resulted in an error."

So in order to fullfil this remark, I can't simply use ENOENT instead of
current NULL, as the caller couldn't distinguish between ENOENT from
of_get_mac_address or ENOENT from NVMEM subsystem.

1. https://patchwork.ozlabs.org/patch/1092243/#2161764

-- ynezz