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

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Date:	Wed, 30 Mar 2016 10:46:23 +0800
From:	Yong Li <sdliyong@...il.com>
To:	unlisted-recipients:; (no To-header on input)
Cc:	linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org
Subject: Re: [PATCH] gpio: pca953x: Use correct u16 value for register word write

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
>