linux-kernel - Re: [PATCH] fb: Rework locking to fix lock ordering on takeover

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Date:	Fri, 04 Jan 2013 14:36:09 +0100
From:	Alexander Holler <holler@...oftware.de>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Borislav Petkov <bp@...en8.de>, Shawn Guo <shawn.guo@...aro.org>,
	Sasha Levin <levinsasha928@...il.com>,
	Cong Wang <xiyou.wangcong@...il.com>,
	Josh Boyer <jwboyer@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Florian Tobias Schandinat <florianSchandinat@....de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fbdev@...r.kernel.org, Bernie Thompson <bernie@...gable.com>,
	Steve Glendinning <steve.glendinning@...well.net>,
	Dave Airlie <airlied@...hat.com>
Subject: Re: [PATCH] fb: Rework locking to fix lock ordering on takeover

Am 04.01.2013 14:25, schrieb Alan Cox:
> On Fri, 04 Jan 2013 13:50:37 +0100
> Alexander Holler <holler@...oftware.de> wrote:
>
>> Am 28.12.2012 13:40, schrieb Borislav Petkov:
>>> On Fri, Dec 28, 2012 at 07:50:27PM +0800, Shawn Guo wrote:
>>>> +1
>>>>
>>>> http://thread.gmane.org/gmane.linux.kernel/1413953/focus=1415070
>>>
>>> Cool, works fine here too. Is Linus on CC? (/me checks.. ) Yes he is,
>>> good.
>>>
>>> Linus, Alan's patch works at least in 2 cases, you might consider
>>> picking it up directly since the fb maintainer is absent, reportedly.
>>
>>
>> Btw. I think all the usb-fb's (udlfb, smscufx and udl) are broken, at
>> least on ARM(v5). When I have linked in udlfb the following happens on
>> boot (with an attached USB-LCD and with or without the "Rework locking
>> patch):
>
> They are broken if used as the system console (has been known for years).

Ah. Thats why I didn't see it before. Usually I've used the serial as 
system console. So thats why it worked before. ;)

> Perhaps your x86 test has the system console still on another device ?

Exactly thats the case. Thanks for pointing it out.

> For the udl layer it shouldn't matter as Dave Airlie wrote a DRM driver
> for udl which obsoletes the old fb layer one and works much better
> (although the error handling is still totally broken and leaks like a
> sieve if it fails)
>
> Fixing the console isn't that difficult - you just need to make your
> device queue the console I/O to a worker thread of some kind. We don't

That is what I wanted to try next. ;)

> want to do that by default because we want to get the messages out
> reliably and immediately on saner hardware. Given there are several
> such cases a general helper and a console "I am crap" flag might be better
> than hacking each driver.

All those drivers look very similiar. I will see if I'm successfull in 
writing such an IamCrapHelper. Might need some time, but I will post a 
patch for review, if I've done and tested it. I'm only using the USB-LCD 
on occasion, so it doesn't have high priority for me because I don't 
really need it.

Thanks for the hints.

Regards,

Alexander
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