Re: [PATCH v3 7/7] i2c: designware: Support of controller with IC_EMPTYFIFO_HOLD_MASTER disabled

[Andy Shevchenko]

On Wed, Nov 19, 2025 at 04:05:36PM +0100, Benoît Monin wrote:
> If IC_EMPTYFIFO_HOLD_MASTER_EN parameter is 0, "Stop" and "Repeated Start"
> bits in command register does not exist, thus it is impossible to send
> several consecutive write messages in a single hardware batch. The
> existing implementation worked with such configuration incorrectly:
> all consecutive write messages are joined into a single message without
> any Start/Stop or Repeated Start conditions. For example, the following
> command:
> 
>     i2ctransfer -y 0 w1@0x55 0x00 w1@0x55 0x01
> 
> does the same as
> 
>     i2ctransfer -y 0 w2@0x55 0x00 0x01
> 
> In i2c_dw_msg_is_valid(), we ensure that we do not have such sequence
> of messages requiring a RESTART, aborting the transfer on controller
> that cannot emit them explicitly.
> 
> This behavior is activated by compatible entries because the state of
> the IC_EMPTYFIFO_HOLD_MASTER_EN parameter cannot be detected at runtime.
> Add the compatible entry for Mobileye SoCs needing the workaround.
> 
> There is another possible problem with this controller configuration:
> When the CPU is putting commands to the FIFO, this process must not be
> interrupted because if FIFO buffer gets empty, the controller finishes
> the I2C transaction and generates STOP condition on the bus.
> 
> If we continue writing the remainder of the message to the FIFO, the
> controller will start emitting a new transaction with those data. This
> turns a single a single message into multiple I2C transactions. To
> protect against FIFO underrun, two changes are done:
> 
> First we flag the interrupt with IRQF_NO_THREAD, to prevent it from
> running in a thread on PREEMPT-RT kernel. This ensures that we are not
> interrupted when filling the FIFO as it is very time-senstive. For
> example, being preempted after writing a single byte in the FIFO with
> a 1MHz bus gives us only 18µs before an underrun.
> 
> Second in i2c_dw_process_transfer(), we abort if a STOP is detected
> while a read or a write is in progress. This can occur when processing
> a message larger than the FIFO. In that case the message is processed in
> parts, and rely on the TX EMPTY interrupt to refill the FIFO when it gets
> below a threshold. If servicing this interrupt is delayed for too long,
> it can trigger a FIFO underrun, thus an unwanted STOP.

...

>  #define ACCESS_NO_IRQ_SUSPEND			BIT(1)
>  #define ARBITRATION_SEMAPHORE			BIT(2)
>  #define ACCESS_POLLING				BIT(3)
> +#define NO_EMPTYFIFO_HOLD_MASTER		BIT(4)

Can we name it

#define ACCESS_NO_EMPTYFIFO_HOLD_MASTER		BIT(4)

?

It will at least keep them under same namespace.

...

> 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?

>  	return true;
>  }

...

> +	/*
> +	 * The first writing to TX FIFO buffer causes transmission start. If
> +	 * IC_EMPTYFIFO_HOLD_MASTER_EN is not set, when TX FIFO gets empty, I2C
> +	 * controller finishes the transaction. If writing to FIFO is

Make the lines more equal by length.

	 * The first writing to TX FIFO buffer causes transmission start.
	 * If IC_EMPTYFIFO_HOLD_MASTER_EN is not set, when TX FIFO gets empty,
	 * I2C controller finishes the transaction. If writing to FIFO is

> +	 * interrupted, FIFO can get empty and the transaction will be finished
> +	 * prematurely. FIFO buffer is filled in IRQ handler, but in PREEMPT_RT
> +	 * kernel IRQ handler by default is executed in thread that can be

> +	 * preempted with another higher priority thread or an interrupt. So,
> +	 * IRQF_NO_THREAD flag is required in order to prevent any preemption
> +	 * when filling the FIFO.

Similarly

	 * preempted with another higher priority thread or an interrupt.
	 * So, IRQF_NO_THREAD flag is required in order to prevent any
	 * preemption when filling the FIFO.


Dunno if the middle part needs to be rewrapped, perhaps so...

> +	 */

...

>  	{ .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,

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

-- 
With Best Regards,
Andy Shevchenko