linux-kernel - Re: [PATCH] [v2] i3c: fix big-endian FIFO transfers

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Message-ID: <ab4b09ba3a332305f5e1e9d456acbecdb492bd85.camel@gmail.com>
Date: Thu, 25 Sep 2025 11:13:39 +0100
From: Nuno Sá <noname.nuno@...il.com>
To: Arnd Bergmann <arnd@...db.de>, Manikanta Guntupalli
	 <manikanta.guntupalli@....com>, Jorge Marques <gastmaier@...il.com>, Arnd
 Bergmann <arnd@...nel.org>
Cc: Alexandre Belloni <alexandre.belloni@...tlin.com>, Jorge Marques	
 <jorge.marques@...log.com>, Wolfram Sang
 <wsa+renesas@...g-engineering.com>,  Frank Li <Frank.Li@....com>,
 "linux-i3c@...ts.infradead.org" <linux-i3c@...ts.infradead.org>, 
 "linux-kernel@...r.kernel.org"	 <linux-kernel@...r.kernel.org>, "git
 (AMD-Xilinx)" <git@....com>, Michal Simek	 <michal.simek@....com>
Subject: Re: [PATCH] [v2] i3c: fix big-endian FIFO transfers

On Thu, 2025-09-25 at 11:35 +0200, Arnd Bergmann wrote:
> On Thu, Sep 25, 2025, at 10:58, Nuno Sá wrote:
> > On Thu, 2025-09-25 at 08:47 +0000, Guntupalli, Manikanta wrote:
> > > > (AMD-Xilinx) <git@....com>; Simek, Michal <michal.simek@....com>
> > > > Subject: Re: [PATCH] [v2] i3c: fix big-endian FIFO transfers
> > > > On Thu, 2025-09-25 at 07:37 +0000, Guntupalli, Manikanta wrote:
> > > > > > i3c@...ts.infradead.org; linux-kernel@...r.kernel.org
> > > > > > Subject: Re: [PATCH] [v2] i3c: fix big-endian FIFO transfers
> > > > > > On Wed, Sep 24, 2025 at 10:18:33PM +0200, Arnd Bergmann wrote:
> > > > 
> > > > I would argue that's something for callers of these functions to care
> > > > about.
> > > If each I3C driver has to handle FIFO endianness individually, it
> > > introduces
> > > unnecessary duplication and overhead across drivers. Centralizing this in
> > > the
> > > FIFO access helpers keeps the logic consistent, avoids repeated
> > > boilerplate,
> > > and reduces the chance of subtle bugs.
> > 
> > I mean, that's what spi and i2c drivers do already. With enum
> > i3c_fifo_endian
> > you're already forcing users to care (or know) about endianism so they might
> > as
> > well just pass the data in the proper order already (not sure if it's such a
> > big
> > 'burden').
> 
> Can you give an example of an spi or i2c driver handles a similar
> situation to the new i3c driver? As far as I can tell, swapping
> the bytes in a FIFO register is very unusual for a hardware design
> and probably a mistake rather than an intentional decision.
> 

I meant that i2c and spi drivers (and I meant on the device side) already are
the ones having to care about putting the data in the proper endianism so that
controllers don't have to care (AFAIK).

But I so see now that the above is kind of unrelated.

> On the other hand, I can find drivers that are obviously wrong
> on big-endian kernels, such as Tegra's i2c_writesl_vi() function
> being unintentionally swapped from i2c_writesl() on big-endian.
> 
> For the i3c helper, I think Jorge's current version with my
> fix should work for every normal driver, and I would not
> want to make it more complicated for an obscure case. The
> version for the AMD driver can just be in that driver, or
> it could be a separate function name in the common header
> if there is a chance we'll need it again.
> 

I do agree with the above.

- Nuno Sá

>       Arnd