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

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Message-ID: <aSWPOJca9dOz2x6B@smile.fi.intel.com>
Date: Tue, 25 Nov 2025 13:12:56 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Benoît Monin <benoit.monin@...tlin.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

On Tue, Nov 25, 2025 at 11:45:35AM +0100, Benoît Monin wrote:
> 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:

[...]

> > > +	/*
> > > +	 * 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.

OK!

> > >  	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.

USB DWC3 driver uses properties for that. We really should go away from this
MODEL_*/FLAG_* legacy approach in the driver. Either compatible string or
a specific property would be better than inventing yet another no so scalable
flag.

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

-- 
With Best Regards,
Andy Shevchenko