linux-kernel - Re: [net-next PATCH] net: phy: aquantia: make mailbox interface4 lsw addr mask more specific

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Message-ID: <655d3e2b.df0a0220.50550.b235@mx.google.com>
Date:   Wed, 22 Nov 2023 00:32:56 +0100
From:   Christian Marangi <ansuelsmth@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Robert Marko <robimarko@...il.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel test robot <lkp@...el.com>
Subject: Re: [net-next PATCH] net: phy: aquantia: make mailbox interface4 lsw
 addr mask more specific

On Tue, Nov 21, 2023 at 03:08:59PM -0800, Jakub Kicinski wrote:
> On Mon, 20 Nov 2023 20:35:04 +0100 Christian Marangi wrote:
> > It seems some arch (s390) require a more specific mask for FIELD_PREP
> > and doesn't like using GENMASK(15, 2) for u16 values.
> > 
> > Fix the compilation error by adding the additional mask for the BITS
> > that the PHY ignore and AND the passed addr with the real mask that the
> > PHY will parse for the mailbox interface 4 addr to make sure extra
> > values are correctly removed.
> 
> Ah. Um. Pff. Erm. I'm not sure.
> 
> Endianness is not my strong suit but this code:
> 
> 	/* PHY expect addr in LE */
> 	addr = (__force u32)cpu_to_le32(addr); 
> 
> 	/* ... use (u16)(addr)       */
> 	/* ... use (u16)(addr >> 16) */
> 
> does not make sense to me.
> 
> You're operating on register values here, there is no endian.
> Endian only exists when you store or load from memory. IOW, this:
> 
> 	addr = 0x12345678;
> 	print((u16)addr);
> 	print(addr >> 16);
> 
> will print the same exact thing regardless of the CPU endian.
> 
> Why did you put the byte swap in there?

the 2 addr comes from a define

#define DRAM_BASE_ADDR		0x3FFE0000
#define IRAM_BASE_ADDR		0x40000000

it wasn't clear to me if on BE these addrs gets saved differently or
not. PHY wants the addr in LE.

On testing by removing the cpu_to_le32 the error is correctly removed!

I guess on BE the addr was actually swapped and FIELD_GET was correctly
warning (and failing) as data was missing in applying the mask.

If all of this makes sense, will send a followup patch that drop the
cpu_to_le32 and also the other in the bottom that does cpu_to_be32 (to a
__swab32 as FW is LE and mailbox calculate CRC in BE)

-- 
	Ansuel