Re: [Patch, RFC] Make struct fb_info ref-counted with kref

[Guennadi Liakhovetski]

On Mon, 20 Sep 2010, Florian Tobias Schandinat wrote:

> Hi Bruno,
> 
> Bruno Prémont schrieb:
> > Hi Florian,
> > 
> > On Sun, 19 September 2010 Florian Tobias Schandinat
> > <FlorianSchandinat@xxxxxx> wrote:
> > > Hi,
> > > 
> > > Bruno Prémont schrieb:
> > > > For USB-attached (or other hot-(un)pluggable) framebuffers the current
> > > > fbdev infrastructure is not very helpful. Each such driver currently
> > > > needs to perform the ref-counting on its own in .fbops.fb_open and
> > > > .fbops.fb_release callbacks.
> > > I agree. This is a great idea even for non-hot-(un)pluggable framebuffers.

Here's another custom fbdev refcounting example:

http://thread.gmane.org/gmane.linux.ports.sh.devel/8841

> > Yes, things like drmfb and drivers of multi-head capable framebuffer
> > drivers would certainly appreciate as well, but they will probably also
> > want to care about users (of fb_info.screen_base).
> 
> I don't see any special users of fb_info.screen_base. It's only used for
> software fallbacks of acceleration functions and fb_read/fb_write (which I'd
> consider rare to fb_mmap). The bad thing of screen_base is that it can make
> viafb try to map up to 512MB on 32 bit systems...
> Much more painful IMHO are the mmaped areas in userspace which essentially
> prevent moving around of the screen framebuffer inside the video ram.
> 
> > > > If you have concerns regarding the API changes, please let me know.
> > > Uhm, I'm not really happy with what we count. With the old method you
> > > mentioned we ref-counted framebuffer users, after your patch it's more
> > > counting users + uses. This might be okay as we usually are interested
> > > whether the ref_count is 0 or not but it doesn't look right if we modify
> > > the refcount during nearly every framebuffer operation. Wouldn't it be
> > > sufficient to do the refcounting in fb_open & fb_release operation + in
> > > fbcon where open&release are done?
> > 
> > Well I'm more for counting the uses, (especially as the aim is to not
> > force the driver to look exactly when it can free the fb_info struct).
> > If the driver needs to know about active users (e.g. for handling memory
> > reorganization on mode change or the like) that would remain driver's job.
> 
> I don't see how your counting would influence the time fb_info is freed. It is
> freed when the last reference is gone but the last remaining reference is
> always  a user reference either from the framebuffer itself or from any user.
> But all users have to keep the framebuffer open to do anything with it
> therfore the last thing they do is releasing the framebuffer. So I do not
> really understand your reasoning, for me counting the users + uses is more
> error prone than just the users. But that's not important for me as I'm only
> interested in whether the count is 0, 1 or more (want to turn off the screen
> if there are no active [=1] users) which is the same regardless on what you
> count. So if you really want to stick to your way of counting, that's no
> problem for me.

In the above mentioned patch I had to distinguish between fbcon and 
user-space users. The reason is, that I can force fbcon to reconfigure by 
sending a FB_EVENT_MODE_CHANGE(_ALL) notification, whereas userspace users 
cannot be asynchronously notified, so, I have to wait until last of them 
releases the framebuffer. Do I understand it right, that the proposed API 
wouldn't provide such a distinction? Of course, the optimal solution would 
be to design and implement a mechanism to notify framebuffer users about a 
changed fb configuration, but we're not that far yet...

Thanks
Guennadi

> 
> > > > diff --git a/drivers/video/fbsysfs.c b/drivers/video/fbsysfs.c
> > > > index 0a08f13..be5f342 100644
> > > > --- a/drivers/video/fbsysfs.c
> > > > +++ b/drivers/video/fbsysfs.c
> > > > @@ -58,6 +58,7 @@ struct fb_info *framebuffer_alloc(size_t size, struct
> > > > device *dev)
> > > >  		info->par = p + fb_info_size;
> > > >   	info->device = dev;
> > > > +	kref_init(&info->refcount);
> > > As far as I know there exist framebuffer drivers which do not call
> > > framebuffer_alloc but contain their own fb_info. I guess these would be
> > > broken as well.
> > 
> > For those it would be better to switch them to be using framebuffer_alloc.
> 
> I don't see any argument against this.
> 
> 
> Thanks,
> 
> Florian Tobias Schandinat
> 
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---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/
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