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Subject: Re: [PATCH v3 2/3] i2c: smbus: use get/put_unaligned_le16 when
 working with word data
To:     Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Wolfram Sang <wsa@the-dreams.de>
Cc:     linux-iio@vger.kernel.orgi, linux-i2c@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20191112203132.163306-1-dmitry.torokhov@gmail.com>
 <20191112203132.163306-3-dmitry.torokhov@gmail.com>
From:   Luca Ceresoli <luca@lucaceresoli.net>
Message-ID: <099e8f9c-354a-8756-a79b-e66c72d36aa7@lucaceresoli.net>
Date:   Wed, 13 Nov 2019 10:47:42 +0100
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Hi Dmitry,

On 12/11/19 21:31, Dmitry Torokhov wrote:
> It is potentially more performant, and also shows intent more clearly,
> to use get_unaligned_le16() and put_unaligned_le16() when working with
> word data.
> 
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> 
> ---
> 
> Changes in v3:
> - split put_unaligned_le16 into a separate patch
> - more call sites converted to get/put_unaligned_le16
> 
>  drivers/i2c/i2c-core-smbus.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/i2c/i2c-core-smbus.c b/drivers/i2c/i2c-core-smbus.c
> index f8708409b4dbc..7b4e2270eeda1 100644
> --- a/drivers/i2c/i2c-core-smbus.c
> +++ b/drivers/i2c/i2c-core-smbus.c
> @@ -15,6 +15,7 @@
>  #include <linux/i2c.h>
>  #include <linux/i2c-smbus.h>
>  #include <linux/slab.h>
> +#include <asm/unaligned.h>
>  
>  #include "i2c-core.h"
>  
> @@ -370,8 +371,7 @@ static s32 i2c_smbus_xfer_emulated(struct i2c_adapter *adapter, u16 addr,
>  			msg[1].len = 2;
>  		else {
>  			msg[0].len = 3;
> -			msgbuf0[1] = data->word & 0xff;
> -			msgbuf0[2] = data->word >> 8;
> +			put_unaligned_le16(data->word, msgbuf0 + 1);
>  		}
>  		break;
>  	case I2C_SMBUS_PROC_CALL:
> @@ -379,8 +379,7 @@ static s32 i2c_smbus_xfer_emulated(struct i2c_adapter *adapter, u16 addr,
>  		read_write = I2C_SMBUS_READ;
>  		msg[0].len = 3;
>  		msg[1].len = 2;
> -		msgbuf0[1] = data->word & 0xff;
> -		msgbuf0[2] = data->word >> 8;
> +		put_unaligned_le16(data->word, msgbuf0 + 1);
>  		break;
>  	case I2C_SMBUS_BLOCK_DATA:
>  		if (read_write == I2C_SMBUS_READ) {
> @@ -487,7 +486,7 @@ static s32 i2c_smbus_xfer_emulated(struct i2c_adapter *adapter, u16 addr,
>  			break;
>  		case I2C_SMBUS_WORD_DATA:
>  		case I2C_SMBUS_PROC_CALL:
> -			data->word = msgbuf1[0] | (msgbuf1[1] << 8);
> +			data->word = get_unaligned_le16(msgbuf1);

Well, msgbuf1 cannot be unaligned, so it looks like you just need to
convert little endian to host endian. Perhaps __le16_to_cpu(msgbuf1) is
better (and equally or more efficient).

> @@ -648,8 +647,7 @@ s32 i2c_smbus_read_i2c_block_data_or_emulated(const struct i2c_client *client,
>  			status = i2c_smbus_read_word_data(client, command + i);
>  			if (status < 0)
>  				return status;
> -			bytes[i] = status & 0xff;
> -			bytes[i + 1] = status >> 8;
> +			put_unaligned_le16(status, values + i);
>  			i += 2;
>  		}

I've been pondering on this one, because 'i' is always an even number.
But, depending on the caller, 'values' could be unaligned, thus
put_unaligned_le16() is OK here.

-- 
Luca