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

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Message-ID: <aLiVHw4WQW69A5qL@smile.fi.intel.com>
Date: Wed, 3 Sep 2025 22:21:03 +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 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().

-- 
With Best Regards,
Andy Shevchenko