linux-kernel - Re: [PATCH v3 7/7] i2c: designware: Support of controller with IC_EMPTYFIFO_HOLD

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Message-ID: <3583116.sQuhbGJ8Bu@benoit.monin>
Date: Tue, 25 Nov 2025 11:45:35 +0100
From: Benoît Monin <benoit.monin@...tlin.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Andi Shyti <andi.shyti@...nel.org>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>,
 Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
 Mika Westerberg <mika.westerberg@...ux.intel.com>,
 Jan Dabros <jsd@...ihalf.com>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Clark Williams <clrkwllms@...nel.org>, Steven Rostedt <rostedt@...dmis.org>,
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
 Gregory CLEMENT <gregory.clement@...tlin.com>,
 Théo Lebrun <theo.lebrun@...tlin.com>,
 Tawfik Bayouk <tawfik.bayouk@...ileye.com>,
 Vladimir Kondratiev <vladimir.kondratiev@...ileye.com>,
 Dmitry Guzman <dmitry.guzman@...ileye.com>, linux-i2c@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev
Subject:
 Re: [PATCH v3 7/7] i2c: designware: Support of controller with
 IC_EMPTYFIFO_HOLD_MASTER disabled

Hi Andy,

Thanks for the reviews.
(ack on other comments)

On Wednesday, 19 November 2025 at 21:15:16 CET, Andy Shevchenko wrote:
> On Wed, Nov 19, 2025 at 04:05:36PM +0100, Benoît Monin wrote:
[...]

> > i2c_dw_msg_is_valid(struct dw_i2c_dev *dev, const struct i2c_msg *msgs, size_t i
> 
> > +	/*
> > +	 * Make sure we don't need explicit RESTART between two messages
> > +	 * in the same direction for controllers that cannot emit them.
> > +	 */
> > +	if (dev->flags & NO_EMPTYFIFO_HOLD_MASTER &&
> > +	    (msgs[idx - 1].flags & I2C_M_RD) == (msgs[idx].flags & I2C_M_RD)) {
> > +		dev_err(dev->dev, "cannot emit RESTART\n");
> > +		return false;
> > +	}
> 
> Ah, Now I see the point of checking the idx first, but can we rather call it
> with idx >= 1 to begin with?
> 
We would still have to check it when calling i2c_dw_msg_is_valid(), as the
first message after a STOP don't have any limitation. It is not just for
protecting against an out-of-bound access to msgs. The validity of a
message is in relation to the previous message in the same transaction.

I will change the comment to make this clearer.

> >  	return true;
> >  }
[...]

> >  	{ .compatible = "baikal,bt1-sys-i2c", .data = (void *)MODEL_BAIKAL_BT1 },
> > +	{ .compatible = "mobileye,eyeq6lplus-i2c", .data = (void *)NO_EMPTYFIFO_HOLD_MASTER },
> 
> Are you expecting more with this? I would rather use a compatible matching
> instead of the flag,
> 
The IC_EMPTYFIFO_HOLD_MASTER_EN parameter is part of the DesignWare IP, it
is not specific to Mobileye. Given that typical i2c accesses (single read,
single write, write-then-read) work on non-PREMPT_RT without this patch, I
suspect there are other controllers that lack the ability to hold the clock
when the FIFO is empty that could benefit from this flag.

> >  	{ .compatible = "mscc,ocelot-i2c", .data = (void *)MODEL_MSCC_OCELOT },
> >  	{ .compatible = "snps,designware-i2c" },
> 
> 

Best regards,
-- 
Benoît Monin, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com