linux-kernel - Re: [PATCH v1] asm-generic: introduce be56 unaligned accessors

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Message-Id: <758e1d68-3a27-4d64-8c45-da829ed5904a@app.fastmail.com>
Date: Mon, 07 Oct 2024 14:44:37 +0000
From: "Arnd Bergmann" <arnd@...db.de>
To: Marius.Cristea@...rochip.com, "David Laight" <David.Laight@...lab.com>
Cc: Linux-Arch <linux-arch@...r.kernel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] asm-generic: introduce be56 unaligned accessors

On Mon, Oct 7, 2024, at 14:40, Marius.Cristea@...rochip.com wrote:
> On Sun, 2024-09-29 at 21:16 +0000, David Laight wrote:
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>> From: marius.cristea@...rochip.com
>> > Sent: 27 September 2024 09:36
>> > 
>> > The PAC194X, IIO driver, is using some unaligned 56 bit registers.
>> > Provide some helper accessors in preparation for the new driver.
>> 
>> Someone please shoot the hardware engineer ;-)
>> 
>> Do separate unaligned access of the first 4 bytes and last four
>> bytes.
>> It can't matter if the middle byte is accessed twice.
>> 
>> Or, for reads read 8 bytes from 'p & ~1ul' and then fixup
>> the value.
>> 
>
> Do you recommend me to drop this patch?
>
> I know that there are some "workarounds", but those didn't "looks"
> nice. I was using that function locally and I got a suggestion from the
> IIO subsystem maintainer to move it into the kernel in order for others
> to used it.

My feeling is that this is too specific to one driver, I don't
expect it to be shared much. I also suspect that most 56-bit
integers in data structures are actually always part of a naturally
aligned 64-bit word. If that is the case here, the driver can
probably better access it as a normal 64-bit number and mask
out the upper 56 bits using the include/linux/bitfield.h helpers.

        Arnd