netdev - Re: [PATCH net] net: enetc: Correct endianness handling in _enetc_rd

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Message-ID: <aLiVk0QYg1VUm9tT@smile.fi.intel.com>
Date: Wed, 3 Sep 2025 22:22:59 +0300
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Simon Horman <horms@...nel.org>
Cc: Claudiu Manoil <claudiu.manoil@....com>,
	Vladimir Oltean <vladimir.oltean@....com>,
	Wei Fang <wei.fang@....com>, Clark Wang <xiaoning.wang@....com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Alex Marginean <alexandru.marginean@....com>, imx@...ts.linux.dev,
	netdev@...r.kernel.org
Subject: Re: [PATCH net] net: enetc: Correct endianness handling in
 _enetc_rd_reg64

On Wed, Sep 03, 2025 at 10:21:03PM +0300, Andy Shevchenko wrote:
> On Tue, Jun 24, 2025 at 05:35:12PM +0100, Simon Horman wrote:
> > enetc_hw.h provides two versions of _enetc_rd_reg64.
> > One which simply calls ioread64() when available.
> > And another that composes the 64-bit result from ioread32() calls.
> > 
> > In the second case the code appears to assume that each ioread32() call
> > returns a little-endian value. However both the shift and logical or
> > used to compose the return value would not work correctly on big endian
> > systems if this were the case. Moreover, this is inconsistent with the
> > first case where the return value of ioread64() is assumed to be in host
> > byte order.
> > 
> > It appears that the correct approach is for both versions to treat the
> > return value of ioread*() functions as being in host byte order. And
> > this patch corrects the ioread32()-based version to do so.
> > 
> > This is a bug but would only manifest on big endian systems
> > that make use of the ioread32-based implementation of _enetc_rd_reg64.
> > While all in-tree users of this driver are little endian and
> > make use of the ioread64-based implementation of _enetc_rd_reg64.
> > Thus, no in-tree user of this driver is affected by this bug.

...

> > @@ -507,7 +507,7 @@ static inline u64 _enetc_rd_reg64(void __iomem *reg)
> >  		tmp = ioread32(reg + 4);
> >  	} while (high != tmp);
> >  
> > -	return le64_to_cpu((__le64)high << 32 | low);
> > +	return (u64)high << 32 | low;
> >  }
> 
> Description and the visible context rings a bell like this is probably a
> reimplementation of ioread64_lo_hi().

And important to add, if the respective (-lo-hi.h) is included, ioread64()
automatically will be ioread64_lo_hi() and hence code can drop all these custom
calls, but again, I haven't looked into it for the details.

-- 
With Best Regards,
Andy Shevchenko