linux-kernel - Re: [PATCH] usb: gadget: f_uac1: Convert use of __constant_cpu_to_le16 to cpu_to

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Date:	Mon, 24 Aug 2015 16:12:29 +0530
From:	Vaishali Thakkar <vthakkar1994@...il.com>
To:	David Laight <David.Laight@...lab.com>
Cc:	Felipe Balbi <balbi@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: gadget: f_uac1: Convert use of __constant_cpu_to_le16
 to cpu_to_le16

On Mon, Aug 24, 2015 at 2:29 PM, David Laight <David.Laight@...lab.com> wrote:
> From: Vaishali Thakkar [mailto:vthakkar1994@...il.com]
>> Sent: 22 August 2015 02:57
> ...
>> >> -     .bcdADC =               __constant_cpu_to_le16(0x0100),
>> >> -     .wTotalLength =         __constant_cpu_to_le16(UAC_DT_TOTAL_LENGTH),
>> >> +     .bcdADC =               cpu_to_le16(0x0100),
>> >> +     .wTotalLength =         cpu_to_le16(UAC_DT_TOTAL_LENGTH),
>> >
>> > Have you test compiled this on a big-endian system?
>> > My gut feeling is that is fails.
>>
>> No. I have tested it on little-endian system only. But I'll
>> be really surprised if this will fail. Can you please tell me
>> if I am missing something in this particular case or same
>> applies for other cases because most of the cases like
>> __constant_<foo> are already converted to <foo>?
>>
>> As far as I know, if the argument is a constant the
>> conversion happens at compile time. And unfolding both
>> definitions returns to same expression. Still I am trying if
>> someone can test it for me on big endian system.
>
> Flip one to cpu_to_be16() and see if it still compiles.

Yes. It still compiles.

> Static initialisers and case labels can be expressions, but the
> expression itself must only contain constants.
> So it needs to be constant regardless of the value of any constants.
> If it contains 'a ? t : f' then both 't' and 'f' must be constant.
>
> In code, if 'a' is constant the optimiser discards one of 't' or 'f'.
> I'm not sure what happens for non-static initialisers (they generate
> odd code at the best of times).
>
>         David
>



-- 
Vaishali
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