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Message-ID: <ZO8wz9KwCbSHATFm@shell.armlinux.org.uk>
Date: Wed, 30 Aug 2023 13:06:39 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Andrew Lunn <andrew@...n.ch>
Cc: "Radu Pirea (OSS)" <radu-nicolae.pirea@....nxp.com>,
Sabrina Dubroca <sd@...asysnail.net>, hkallweit1@...il.com,
davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
pabeni@...hat.com, richardcochran@...il.com,
sebastian.tobuschat@....com, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [RFC net-next v2 3/5] net: phy: nxp-c45-tja11xx add MACsec
support
On Fri, Aug 25, 2023 at 03:50:06PM +0200, Andrew Lunn wrote:
> > > > > +static bool nxp_c45_rx_sc_valid(struct nxp_c45_secy *phy_secy,
> > > > > + struct macsec_rx_sc *rx_sc)
> > > > > +{
> > > > > + u16 port = (__force u64)rx_sc->sci >> (ETH_ALEN * 8);
> > > >
> > > > u64 sci = be64_to_cpu((__force __be64)rx_sc->sci);
> > >
> > > why is the __force needed? What happens with a normal cast?
> > >
> >
> > Sparse will print warnings if __force is missing.
>
> What is the warning? I just want to make sure __force is the correct
> solution, not that something has the wrong type and we should be
> fixing a design issue.
Hi Andrew,
rx_sc->sci is sci_t, which is defined as:
typedef u64 __bitwise sci_t;
Sparse documentation (Documentation/dev-tools/sparse.rst) states that:
"__bitwise" is a type attribute, so you have to do something like this::
...
which makes PM_SUSPEND and PM_RESUME "bitwise" integers (the "__force"
is there because sparse will complain about casting to/from a bitwise
type, but in this case we really _do_ want to force the conversion).
So basically, sci is a bitwise type, which means sparse gives it
special properties that ensures it can only be operated on using
similarly typed integers.
So, those __force casts are needed to convert sci to something else.
--
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