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Date:   Fri, 29 Sep 2023 19:08:57 +0300
From:   Serge Semin <fancer.lancer@gmail.com>
To:     Tam Nguyen <tamnguyenchi@os.amperecomputing.com>
Cc:     linux-kernel@vger.kernel.org, linux-i2c@vger.kernel.org,
        patches@amperecomputing.com, jarkko.nikula@linux.intel.com,
        andriy.shevchenko@linux.intel.com, mika.westerberg@linux.intel.com,
        jsd@semihalf.com, chuong@os.amperecomputing.com,
        darren@os.amperecomputing.com, stable@vger.kernel.org
Subject: Re: [PATCH v1] i2c: designware: Disable TX_EMPTY irq while waiting
 for block length byte
Message-ID: <avd7jhwexehgbvi6euzdwvf5zvqqgjx4ozo6uxu2qpmlarvva3@sgkce3rvovwk>
References: <20230929035356.6435-1-tamnguyenchi@os.amperecomputing.com>
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In-Reply-To: <20230929035356.6435-1-tamnguyenchi@os.amperecomputing.com>
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On Fri, Sep 29, 2023 at 10:53:56AM +0700, Tam Nguyen wrote:
> During SMBus block data read process, we have seen high interrupt rate
> because of TX_EMPTY irq status while waiting for block length byte (the
> first data byte after the address phase). The interrupt handler does not
> do anything because the internal state is kept as STATUS_WRITE_IN_PROGRESS.
> Hence, we should disable TX_EMPTY irq until I2C DW receives first data
> byte from I2C device, then re-enable it.
> 
> It takes 0.789 ms for host to receive data length from slave.
> Without the patch, i2c_dw_isr is called 99 times by TX_EMPTY interrupt.
> And it is none after applying the patch.
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Chuong Tran <chuong@os.amperecomputing.com>
> Signed-off-by: Tam Nguyen <tamnguyenchi@os.amperecomputing.com>
> ---
>  drivers/i2c/busses/i2c-designware-master.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/i2c/busses/i2c-designware-master.c b/drivers/i2c/busses/i2c-designware-master.c
> index 55ea91a63382..2152b1f9b27c 100644
> --- a/drivers/i2c/busses/i2c-designware-master.c
> +++ b/drivers/i2c/busses/i2c-designware-master.c
> @@ -462,6 +462,13 @@ i2c_dw_xfer_msg(struct dw_i2c_dev *dev)

>  		if (buf_len > 0 || flags & I2C_M_RECV_LEN) {
>  			/* more bytes to be written */
>  			dev->status |= STATUS_WRITE_IN_PROGRESS;
> +			/*
> +			 * In I2C_FUNC_SMBUS_BLOCK_DATA case, there is no data
> +			 * to send before receiving data length from slave.
> +			 * Disable TX_EMPTY while waiting for data length byte
> +			 */
> +			if (flags & I2C_M_RECV_LEN)
> +				intr_mask &= ~DW_IC_INTR_TX_EMPTY;

Is it possible to reduce the indentations level? Like this:

		/*
		 * Because we don't know the buffer length in the
		 * I2C_FUNC_SMBUS_BLOCK_DATA case, we can't stop the
		 * transaction here. Also disable the TX_EMPTY IRQ
		 * while waiting for the data length byte to avoid the
		 * bogus interrupts flood.
		 */
		if (flags & I2C_M_RECV_LEN) {
  			dev->status |= STATUS_WRITE_IN_PROGRESS;
			intr_mask &= ~DW_IC_INTR_TX_EMPTY;
			break;
		} else if (buf_len > 0) {
  			/* more bytes to be written */
  			dev->status |= STATUS_WRITE_IN_PROGRESS;
			break;
		} else {
			dev->status &= ~STATUS_WRITE_IN_PROGRESS;
		}

>  			break;
>  		} else
>  			dev->status &= ~STATUS_WRITE_IN_PROGRESS;
> @@ -485,6 +492,7 @@ i2c_dw_recv_len(struct dw_i2c_dev *dev, u8 len)
>  {
>  	struct i2c_msg *msgs = dev->msgs;
>  	u32 flags = msgs[dev->msg_read_idx].flags;

> +	u32 intr_mask;
>  
>  	/*
>  	 * Adjust the buffer length and mask the flag
> @@ -495,6 +503,11 @@ i2c_dw_recv_len(struct dw_i2c_dev *dev, u8 len)
>  	msgs[dev->msg_read_idx].len = len;
>  	msgs[dev->msg_read_idx].flags &= ~I2C_M_RECV_LEN;
>  
> +	/* Re-enable TX_EMPTY interrupt. */
> +	regmap_read(dev->map, DW_IC_INTR_MASK, &intr_mask);
> +	intr_mask |= DW_IC_INTR_TX_EMPTY;
> +	regmap_write(dev->map, DW_IC_INTR_MASK, intr_mask);

1. What about just:
	regmap_update_bits(dev->map, DW_IC_INTR_MASK, DW_IC_INTR_TX_EMPTY,
			   DW_IC_INTR_TX_EMPTY);

2. The in-situ comment is pointless because the statement already
implies the IRQ re-enabling. I suggest to add more details of _why_
the IRQ needs to be re-enabled (what is supposed to be done after it's
re-enabled?).

-Serge(y)

> +
>  	return len;
>  }
>  
> -- 
> 2.25.1
>