linux-kernel - RE: [PATCH V7 3/4] i3c: master: Add endianness support for i3c_readl_fifo() and i3c_writel

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Message-ID:
 <DM4PR12MB61090F307DBA2B99AFC93B168C1FA@DM4PR12MB6109.namprd12.prod.outlook.com>
Date: Thu, 25 Sep 2025 09:26:34 +0000
From: "Guntupalli, Manikanta" <manikanta.guntupalli@....com>
To: Arnd Bergmann <arnd@...db.de>, "git (AMD-Xilinx)" <git@....com>, "Simek,
 Michal" <michal.simek@....com>, Alexandre Belloni
	<alexandre.belloni@...tlin.com>, Frank Li <Frank.Li@....com>, Rob Herring
	<robh@...nel.org>, "krzk+dt@...nel.org" <krzk+dt@...nel.org>, Conor Dooley
	<conor+dt@...nel.org>, Przemysław Gaj <pgaj@...ence.com>,
	Wolfram Sang <wsa+renesas@...g-engineering.com>,
	"tommaso.merciai.xr@...renesas.com" <tommaso.merciai.xr@...renesas.com>,
	"quic_msavaliy@...cinc.com" <quic_msavaliy@...cinc.com>, "S-k, Shyam-sundar"
	<Shyam-sundar.S-k@....com>, Sakari Ailus <sakari.ailus@...ux.intel.com>,
	"'billy_tsai@...eedtech.com'" <billy_tsai@...eedtech.com>, Kees Cook
	<kees@...nel.org>, "Gustavo A. R. Silva" <gustavoars@...nel.org>, Jarkko
 Nikula <jarkko.nikula@...ux.intel.com>, Jorge Marques
	<jorge.marques@...log.com>, "linux-i3c@...ts.infradead.org"
	<linux-i3c@...ts.infradead.org>, "devicetree@...r.kernel.org"
	<devicetree@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Linux-Arch <linux-arch@...r.kernel.org>,
	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
CC: "Pandey, Radhey Shyam" <radhey.shyam.pandey@....com>, "Goud, Srinivas"
	<srinivas.goud@....com>, "Datta, Shubhrajyoti" <shubhrajyoti.datta@....com>,
	"manion05gk@...il.com" <manion05gk@...il.com>
Subject: RE: [PATCH V7 3/4] i3c: master: Add endianness support for
 i3c_readl_fifo() and i3c_writel_fifo()

[Public]

Hi,

> -----Original Message-----
> From: Arnd Bergmann <arnd@...db.de>
> Sent: Wednesday, September 24, 2025 9:13 PM
> To: Guntupalli, Manikanta <manikanta.guntupalli@....com>; git (AMD-Xilinx)
> <git@....com>; Simek, Michal <michal.simek@....com>; Alexandre Belloni
> <alexandre.belloni@...tlin.com>; Frank Li <Frank.Li@....com>; Rob Herring
> <robh@...nel.org>; krzk+dt@...nel.org; Conor Dooley <conor+dt@...nel.org>;
> Przemysław Gaj <pgaj@...ence.com>; Wolfram Sang <wsa+renesas@...g-
> engineering.com>; tommaso.merciai.xr@...renesas.com;
> quic_msavaliy@...cinc.com; S-k, Shyam-sundar <Shyam-sundar.S-k@....com>;
> Sakari Ailus <sakari.ailus@...ux.intel.com>; 'billy_tsai@...eedtech.com'
> <billy_tsai@...eedtech.com>; Kees Cook <kees@...nel.org>; Gustavo A. R. Silva
> <gustavoars@...nel.org>; Jarkko Nikula <jarkko.nikula@...ux.intel.com>; Jorge
> Marques <jorge.marques@...log.com>; linux-i3c@...ts.infradead.org;
> devicetree@...r.kernel.org; linux-kernel@...r.kernel.org; Linux-Arch <linux-
> arch@...r.kernel.org>; linux-hardening@...r.kernel.org
> Cc: Pandey, Radhey Shyam <radhey.shyam.pandey@....com>; Goud, Srinivas
> <srinivas.goud@....com>; Datta, Shubhrajyoti <shubhrajyoti.datta@....com>;
> manion05gk@...il.com
> Subject: Re: [PATCH V7 3/4] i3c: master: Add endianness support for i3c_readl_fifo()
> and i3c_writel_fifo()
>
> On Wed, Sep 24, 2025, at 17:23, Guntupalli, Manikanta wrote:
> >> Subject: Re: [PATCH V7 3/4] i3c: master: Add endianness support for
> >> i3c_readl_fifo() and i3c_writel_fifo()
> >> > }
> >> >
> >> > With this approach, both little-endian and big-endian cases works as expected.
> >>
> >> This version should fix the cases where you have a big-endian kernel
> >> with either I3C_FIFO_BIG_ENDIAN or I3C_FIFO_LITTLE_ENDIAN, as neither
> >> combination does any byte swaps.
> >>
> >> However I'm fairly sure it's still broken for little-endian kernels
> >> when a driver asks for a I3C_FIFO_BIG_ENDIAN conversion, same as v7.
> > We tested using the I3C_FIFO_BIG_ENDIAN flag from the driver on
> > little-endian kernels, and it works as expected.
>
> Can you explain how that works? What I see is that your
> readsl_be()/writesl_be() functions do a byteswap on every four bytes, so the
> bytestream that gets copied to/from the FIFO gets garbled, in particular the final
> (unaligned) bytes of the kernel buffer end up in the higher bytes of the FIFO register
> rather than the first bytes as they do on a big-endian kernel.
>
> Are both the big-endian and little-endian kernels in your tests on microblaze, using
> the upstream version of asm/io.h? Is there a hardware byteswap between the CPU
> local bus and the i3c controller? If there is one, is it set the same way for both
> kernels?
>
To clarify, my testing was performed on the latest upstream kernel on a ZCU102 (Zynq UltraScale+ MPSoC, Cortex-A53, little-endian) with big-endian FIFOs and no bus-level byteswap. For more details, please refer to my reply in Re: [PATCH] [v2] i3c: fix big-endian FIFO transfers.

Please don't take this as negative or aggressive-my intention is purely to learn and ensure it works correctly in all cases.

Thanks,
Manikanta.