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

From: "Arnd Bergmann" <arnd@arndb.de>
To: "Nuno Sá" <noname.nuno@gmail.com>,
	"Manikanta Guntupalli" <manikanta.guntupalli@amd.com>,
	"Jorge Marques" <gastmaier@gmail.com>,
	"Arnd Bergmann" <arnd@kernel.org>
Cc: "Alexandre Belloni" <alexandre.belloni@bootlin.com>,
	"Jorge Marques" <jorge.marques@analog.com>,
	"Wolfram Sang" <wsa+renesas@sang-engineering.com>,
	"Frank Li" <Frank.Li@nxp.com>,
	"linux-i3c@lists.infradead.org" <linux-i3c@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"git (AMD-Xilinx)" <git@amd.com>,
	"Michal Simek" <michal.simek@amd.com>
Subject: Re: [PATCH] [v2] i3c: fix big-endian FIFO transfers
Date: Thu, 25 Sep 2025 11:35:44 +0200	[thread overview]
Message-ID: <91e73a29-96e3-4a52-addb-0cb954f46c04@app.fastmail.com> (raw)
In-Reply-To: <e5a6789230840b31ef0f60ca0a770a4fb266da2a.camel@gmail.com>

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@amd.com>; Simek, Michal <michal.simek@amd.com>
>> > Subject: Re: [PATCH] [v2] i3c: fix big-endian FIFO transfers
>> > On Thu, 2025-09-25 at 07:37 +0000, Guntupalli, Manikanta wrote:
>> > > > i3c@lists.infradead.org; linux-kernel@vger.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.

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.

      Arnd

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