linux-kernel - Re: [PATCH] gpio: pca953x: Use correct u16 value for register word write

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Message-ID: <CADO9-pfuBVA1HW+XrV5oN_6jF2mebi3QeTiro1B8DCk+Em2Yug@mail.gmail.com>
Date:	Wed, 30 Mar 2016 13:01:17 +0800
From:	Yong Li <sdliyong@...il.com>
To:	Phil Reid <preid@...ctromag.com.au>
Cc:	Alexander Stein <alexander.stein@...tec-electronic.com>,
	linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org
Subject: Re: [PATCH] gpio: pca953x: Use correct u16 value for register word write

Or another method is using the below to convert the u8 to u16:
cpu_to_le16(get_unaligned((u16 *) val)), compared with the
i2c_smbus_write_byte_data method, which one is better?

Thanks,
Yong

2016-03-30 10:43 GMT+08:00 Yong Li <sdliyong@...il.com>:
> If use the get_unaligned, below is the code example, but we cannot detect if
> it is big endian or little endian. I would like to use the same write logic
> as PCA957X_TYPE: use the i2c_smbus_write_byte_data API to write two times.
> How do you think about it?
>
> if (big_endian)
> value = get_unaligned_be16(buf);
> else
> value = get_unaligned_le16(buf);
>
> Thanks,
> Yong Li
> 2016-03-30 0:33 GMT+08:00 Phil Reid <preid@...ctromag.com.au>:
>>
>> On 29/03/2016 10:39 PM, Alexander Stein wrote:
>>>
>>> You missed CC'ing Phil (Added for this post)
>>>
>>> On Tuesday 29 March 2016 20:53:58, Yong Li wrote:
>>>>
>>>> Thanks for your comment, I think I can change it to val[0] | (val[1]
>>>> << 8), is it okay ?
>>>
>>>
>>> Mh, currently there is only one caller (device_pca953x_init) which passes
>>> only
>>> 0, 0 or 0xff, 0xff, so endianess is irrelevant. But to be future proof
>>> this
>>> should be done in an endian-safe manner. Though cpu_to_le16p does not
>>> work,
>>> due to same alignment problem as casting to u16*.
>>>
>>
>> I think get_unaligned((u16 *) val) should do the job.
>> There's also get_unaligned_le* get_unaligned_be*
>>
>> --
>> Regards
>> Phil Reid
>>