netdev - Re: [PATCH v1] ethernet: marvell: Use min macro

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Message-ID: <20240901171150.GA23170@kernel.org>
Date: Sun, 1 Sep 2024 18:11:50 +0100
From: Simon Horman <horms@...nel.org>
To: David Laight <David.Laight@...lab.com>
Cc: Yan Zhen <yanzhen@...o.com>,
	"marcin.s.wojtas@...il.com" <marcin.s.wojtas@...il.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	"kuba@...nel.org" <kuba@...nel.org>,
	"pabeni@...hat.com" <pabeni@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"opensource.kernel@...o.com" <opensource.kernel@...o.com>
Subject: Re: [PATCH v1] ethernet: marvell: Use min macro

On Sat, Aug 31, 2024 at 12:39:28PM +0000, David Laight wrote:
> From: Simon Horman
> > Sent: 27 August 2024 18:58
> > 
> > On Tue, Aug 27, 2024 at 06:54:08PM +0100, Simon Horman wrote:
> > > On Tue, Aug 27, 2024 at 07:58:48PM +0800, Yan Zhen wrote:
> > > > Using the real macro is usually more intuitive and readable,
> > > > When the original file is guaranteed to contain the minmax.h header file
> > > > and compile correctly.
> > > >
> > > > Signed-off-by: Yan Zhen <yanzhen@...o.com>
> > > > ---
> > > >  drivers/net/ethernet/marvell/mvneta.c | 3 +--
> > > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
> > > > index d72b2d5f96db..415d2b9e63f9 100644
> > > > --- a/drivers/net/ethernet/marvell/mvneta.c
> > > > +++ b/drivers/net/ethernet/marvell/mvneta.c
> > > > @@ -4750,8 +4750,7 @@ mvneta_ethtool_set_ringparam(struct net_device *dev,
> > > >
> > > >  	if ((ring->rx_pending == 0) || (ring->tx_pending == 0))
> > > >  		return -EINVAL;
> > > > -	pp->rx_ring_size = ring->rx_pending < MVNETA_MAX_RXD ?
> > > > -		ring->rx_pending : MVNETA_MAX_RXD;
> > > > +	pp->rx_ring_size = min(ring->rx_pending, MVNETA_MAX_RXD);
> > >
> > > Given that the type of ring->rx_pending is __32, and MVNETA_MAX_RXD is
> > > a positive value.
> > 
> > Sorry, I hit send to soon. What I wanted to say is:
> > 
> > I think that it is appropriate to use umin() here.
> > Because:
> > 1) As I understand things, the type of MVNETA_MAX_RXD is signed,
> >    but it always holds a positive value
> > 2) ring->rx_pending is unsigned
> 
> Provided MVNETA_MAX_RXD is constant it is fine.
> umin() is only needed for signed variables that can only contain
> non-negative values.
> 
> You only need to use it is the compiler bleats...
> 
> umin(x, y) is safer than min_t(unsigned_type, x, y) because you can't
> get the type wrong.
> If will also generate better code since it never sign extends a
> 32bit value to 64bits (expensive on 32bit).

Hi David,

My understanding of umin() was a bit off - I thought it was
also relevant when one of the arguments is a constant.

Thanks for setting me straight.