linux-kernel - Re: [Patch, RFC] Make struct fb

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Message-ID: <20100921075610.1c016c12@pluto.restena.lu>
Date:	Tue, 21 Sep 2010 07:56:10 +0200
From:	Bruno Prémont <bonbons@...ux-vserver.org>
To:	Florian Tobias Schandinat <FlorianSchandinat@....de>
Cc:	linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Bernie Thompson <bernie@...gable.com>
Subject: Re: [Patch, RFC] Make struct fb_info ref-counted with kref

On Tue, 21 Sep 2010 00:28:27 Florian Tobias Schandinat <FlorianSchandinat@....de> wrote:
> > Tracking if/how often framebuffer is opened as such is a separate thing (though
> > all users that have the framebuffer opened hold a reference to fb_info).
> 
> That's what I said. So as long as refcount <= 1 it does not matter whether you 
> just count on open/release or additionally on every framebuffer operation, just 
> that the later produces more noise.

Hm, I don't count on every framebuffer operation... in most cases
fb_info is provided as function argument, in which case no further
counting is needed as the caller has a valid reference.

With my patch applied refcount for registered but unsed framebuffer was
2 (once for the driver, once for registered_fb entry) and went up to 3
when userspace opened framebuffer. fbcon's usage only incremented
refcount for very short timeframes when effectively using fb_info.

When starting with the FB minor I have to take a new reference.
(though I maybe should check if file's private data is set and use
that reference instead of looking up fb_info by minor as is currently
done)

For fbcon all the references are taken by FB minor (I wondered why
fbcon only remembers index into registered_fb aka minor instead of
fb_info itself)

> So I still don't see any advantage in counting users + uses.
> Please note that I do not object the idea of the patch itself, it's only that I 
> have a different preference on what to count. I only want to express that your 
> way is more complicated than what I would recommend.

I don't think I see how you would do the refcounting... would you just
drop the changes in fb_open() and fb_release()?
Could you describe your approach (with pseudo-code) or the differences
to mine?

Thanks,
Bruno

> But if you want to go on I do not object. As long as the end result works that's 
> okay with me.
> 
> 
> Thanks
> 
> Florian Tobias Schandinat
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