linux-kernel - Re: [PATCH v4 2/4] usb: gadget: add quirk_ep_out_aligned_size field to struct usb

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Date:	Tue, 05 Nov 2013 10:13:58 -0800
From:	David Cohen <david.a.cohen@...ux.intel.com>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	balbi@...com, gregkh@...uxfoundation.org,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/4] usb: gadget: add quirk_ep_out_aligned_size field
 to struct usb_gadget

On 11/05/2013 07:41 AM, Alan Stern wrote:
> On Tue, 5 Nov 2013, David Cohen wrote:
> 
>>>> +static inline size_t usb_ep_align_maxpacketsize(struct usb_ep *ep, size_t len)
>>>> +{
>>>> +	int aligned;
>>>> +
>>>> +	if (ep->desc->bmAttributes & USB_ENDPOINT_XFER_INT)
>>>> +		/*
>>>> +		 * Interrupt eps don't need max packet size to be power of 2,
>>>> +		 * so can't use cheap IS_ALIGNED() macro.
>>>> +		 */
>>>> +		aligned = !(len % ep->desc->wMaxPacketSize);
>>>> +	else
>>>> +		aligned = IS_ALIGNED(len, ep->desc->wMaxPacketSize);
>>>
>>> This isn't on a hot path, and I suspect that the extra "if" will 
>>> require nearly as much time as you save by not doing the division.  You 
>>> might as well always use the % operation.
>>
>> Perhaps if I use unlikely() on 'if' condition instead?
>> Anyway I'll double check the costs of if + IS_ALIGNED vs modulo.
> 
> You're missing the point.  You and I (not to mention anybody who ever
> reads this code in the future) have already wasted more time talking
> about it and trying to understand it than you will ever save by using
> IS_ALIGNED.
> 
> The difference to the computer is minimal at best.  Make things easier 
> for the programmers.

I don't see it as complex :)
But I'm fine with your proposal. I can send new patch dropping
IS_ALIGNED() case.

Br, David Cohen
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