linux-kernel - Re: [PATCH v3 2/3] i2c: smbus: use get/put_unaligned

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Date:   Thu, 14 Nov 2019 10:23:32 +0100
From:   Luca Ceresoli <luca@...aceresoli.net>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Wolfram Sang <wsa@...-dreams.de>, linux-iio@...r.kernel.orgi,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/3] i2c: smbus: use get/put_unaligned_le16 when
 working with word data

Hi Dmitry,

On 13/11/19 20:39, Dmitry Torokhov wrote:
> Hi Luca,
> 
> On Wed, Nov 13, 2019 at 10:47:42AM +0100, Luca Ceresoli wrote:
>> 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@...il.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).
> 
> Well, msgbuf1 (as is any other variable unless adjusted by special
> alignment attribute) is naturally aligned for its own data type (which
> for u8 means it can start at any address), but that does not mean that
> is is aligned for the purpose of storing word data. In fact, the
> preceding msgbuf0 variable is 32 + 3 = 35 bytes long, which means that
> msgbuf1 is starting on an odd address, and is definitely not aligned for
> word access and using __le16_to_cpu to fetch that word will trap on some
> architectures.

Uhm, you're obviously right. And I was probably drunk when I wrote the
opposite! Sorry about that...

Reviewed-by: Luca Ceresoli <luca@...aceresoli.net>

-- 
Luca