Re: [PATCH] usb: gadget: f_uac1: Convert use of __constant_cpu_to_le16 to cpu_to_le16

[Vaishali Thakkar]

On Thu, Aug 20, 2015 at 4:20 PM, David Laight <David.Laight@xxxxxxxxxx> wrote:
> From: Vaishali Thakkar
>> Sent: 19 August 2015 06:31
>> To: Felipe Balbi
>> Cc: Greg Kroah-Hartman; linux-usb@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
>> Subject: [PATCH] usb: gadget: f_uac1: Convert use of __constant_cpu_to_le16 to cpu_to_le16
>>
>> In big endian cases, macro cpu_to_le16 unfolds to __swab16 which
>> provides special case for constants. In little endian cases,
>> __constant_cpu_to_le16 and cpu_to_le16 expand directly to the
>> same expression. So, replace __constant_cpu_to_le16 with
>> cpu_to_le16 with the goal of getting rid of the definition of
>> __constant_cpu_to_le16 completely.
>>
>> The semantic patch that performs this transformation is as follows:
>>
>> @@expression x;@@
>>
>> - __constant_cpu_to_le16(x)
>> + cpu_to_le16(x)
>>
>> Signed-off-by: Vaishali Thakkar <vthakkar1994@xxxxxxxxx>
>> ---
>>  drivers/usb/gadget/function/f_uac1.c | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/usb/gadget/function/f_uac1.c b/drivers/usb/gadget/function/f_uac1.c
>> index 7856b33..5aa8d8a 100644
>> --- a/drivers/usb/gadget/function/f_uac1.c
>> +++ b/drivers/usb/gadget/function/f_uac1.c
>> @@ -57,8 +57,8 @@ static struct uac1_ac_header_descriptor_1 ac_header_desc = {
>>       .bLength =              UAC_DT_AC_HEADER_LENGTH,
>>       .bDescriptorType =      USB_DT_CS_INTERFACE,
>>       .bDescriptorSubtype =   UAC_HEADER,
>> -     .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.

>         David
>



-- 
Vaishali
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