linux-kernel - Re: 2.6.30-rc2-git2: Reported regressions from 2.6.29

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Date:	Tue, 21 Apr 2009 09:47:34 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Laurent Pinchart <laurent.pinchart@...net.be>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Adrian Bunk <bunk@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Natalie Protasevich <protasnb@...il.com>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	Linux ACPI <linux-acpi@...r.kernel.org>,
	Linux PM List <linux-pm@...ts.linux-foundation.org>,
	Linux SCSI List <linux-scsi@...r.kernel.org>,
	video4linux-list@...hat.com, mchehab@...radead.org
Subject: Re: 2.6.30-rc2-git2: Reported regressions from 2.6.29

2009/4/21 Laurent Pinchart <laurent.pinchart@...net.be>:
> On Saturday 18 April 2009 06:51:11 leiming wrote:
>> On Fri, 17 Apr 2009 19:55:29 -0700 (PDT)
>>
>> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>> > > @@ -742,7 +742,7 @@ static int uvc_alloc_urb_buffers(struct
>> > > uvc_video_device *video,
>> > >   /* Buffers are already allocated, bail out. */
>> > >   if (video->urb_size)
>> > > -         return 0;
>> > > +         return DIV_ROUND_UP(video->urb_size, psize);
>> >
>> > I don't think this is right. It should round _down_.
>> >
>> > It's supposed to return 'npackets', but if you pass it a different
>> > packet size than it was passed originally, it can now return a
>> > potentially bigger number than the already allocated buffer, no?
>> >
>> > So I think it should round down (ie use a regular divide). No?
>>
>> Yes,you are correct, please ignore my last reply, and following is
>> the fixed patch.
>
> psize and video->urb_size shouldn't have changed before and after resume,
> otherwise we'll get into trouble anyway. A regular divide and a round-up
> divide should then return the same result. I'll take the regular divide, as it
> will be more efficient.

Yes.

>
>> Thanks.
>>
>> From a3b3d72cdd57a0699fb643b41b78eb7beb211ff5 Mon Sep 17 00:00:00 2001
>> From: Ming Lei <tom.leiming@...il.com>
>> Date: Wed, 15 Apr 2009 22:32:51 +0800
>> Subject: [PATCH] V4L/DVB:usbvideo:fix uvc resume failed(v2)
>>
>> Now urb buffers is not freed before suspend, so uvc_alloc_urb_buffers
>> should return packet counts allocated originally during uvc resume
>> , instead of zero.
>>
>> This version uses round down to return packet counts on Linus's
>> suggestions, or else may lead to buffer destructed if packet size
>> is changed before calling uvc_alloc_urb_buffers() in this kind of
>> case.
>
> The comment is misleading. If the packet size changes we need to reallocate
> the buffers anyway. Have you checked if the packet size (which depends on the
> endpoint being selected) can be changed between suspend and resume, either by
> the uvcvideo driver (I don't think it can) or the USB core ?

The packet size does not change between suspend and resume.  I mean
uvc_alloc_urb_buffers()
still can be used in other cases if buffers was not freed and is
reuesed in future. It seems there is no
such cases in uvcvideo now, but uvc_alloc_urb_buffers() really __can__
 work in such case, isn't it?

IMHO It is only used to allocate or reserve UVC_URBS usb buffers,
which size is video->urb_size, and
npackets can be shortened or enlarged if psize is changed, after all.

Thanks!

-- 
Lei Ming
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