Re: [PATCH] usb: gadget: f_uac1: Convert use of __constant_cpu_to_le16 to cpu_to_le16
From: Vaishali Thakkar
Date: Mon Aug 24 2015 - 06:42:51 EST
On Mon, Aug 24, 2015 at 2:29 PM, David Laight <David.Laight@xxxxxxxxxx> wrote:
> From: Vaishali Thakkar [mailto:vthakkar1994@xxxxxxxxx]
>> 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).
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