[PATCH V2] video: implement a simple framebuffer driver

Laurent Pinchart laurent.pinchart at ideasonboard.com
Tue Apr 30 05:50:56 EDT 2013

Hi Olof,

On Monday 29 April 2013 15:40:44 Olof Johansson wrote:
> On Mon, Apr 29, 2013 at 3:23 PM, Laurent Pinchart wrote:
> > On Tuesday 30 April 2013 00:04:20 Arnd Bergmann wrote:
> >> On Monday 29 April 2013, Laurent Pinchart wrote:
> >> > On Monday 29 April 2013 23:31:30 Tomasz Figa wrote:
> >> > > Good point. Stephen, would it be a problem to make this a KMS driver
> >> > > instead? Old fbdev API could be emulated on top of it, until it goes
> >> > > out of use, couldn't it?
> >> > 
> >> > There's already an fbdev emulation layer in KMS, for such a simple use
> >> > case it will work fine.
> >> 
> >> I suggested the same to Stephen when he first brought up this driver.
> >> Unfortunately his attempt to create a simple KMS driver resulted in a
> >> significantly larger driver than the one he did for the framebuffer
> >> interface. This means that either Stephen did something really wrong in
> >> his attempt, or the KMS interface isn't as good as it should be if we
> >> want to move people away from frame buffer drivers.
> > 
> > Implementing a KMS driver requires a fair amount of code. The DRM/KMS
> > subsystem provides helpers that significantly reduce the driver size.
> > Those helpers have been designed around common use cases found in existing
> > KMS drivers.
> > 
> > I'm pretty sure we will be able to add useful helpers for existing fbdev
> > drivers that will make porting them to KMS pretty easy, but that first
> > requires starting to port drivers to find out what common code could be
> > factored out.
> Meanwhile this tiny driver will allow us to use hardware that can't
> otherwise be used (because the proper graphics drivers for said hardware is
> _also_ stuck behind large reworks, i.e. common display framework, which has
> uncertain progress at this time).

Point taken :-)

> As Stephen mentioned in the original patch, this particular driver is very
> useful for Raspberry Pi. But it is also the best way forward (right now) for
> getting basic upstream usability out of the Samsung ARM-based Chromebook. As
> a matter of fact, as of this weekend linux-next boots on that platform with
> keyboard, trackpad, framebuffer and USB2.0 without a single out-of-tree
> patch. Getting the same functionality out of 3.10 would be very desirable.
> So, I clearly understand the desire to move over to a more modern framework.
> At the same time, there's definite value in merging this small driver while
> that's happening. Holding useful code hostage isn't always very productive.

I want to be clear about this, even though I believe we should put as much 
effort as possible behind KMS drivers, I won't nack this patch. It has real 
use cases and is useful, I'm just concerned it will get abused (but it's far 
from being the only piece of code that could be subject to abuse).


Laurent Pinchart

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