Re: [PATCH v2 1/2] drm/cma-helper: Add multi buffer support for cma fbdev

[Maxime Ripard]

Hi,

On Mon, Feb 13, 2017 at 11:20:51AM +0000, Daniel Stone wrote:
> Hi Maxime,
> 
> On 13 February 2017 at 10:54, Maxime Ripard
> <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote:
> > On Sun, Feb 12, 2017 at 02:28:11PM +0200, Laurent Pinchart wrote:
> >> On Thursday 02 Feb 2017 11:31:56 Maxime Ripard wrote:
> >> > This patch add a config to support to create multi buffer for cma fbdev.
> >> > Such as double buffer and triple buffer.
> >> >
> >> > Cma fbdev is convient to add a legency fbdev. And still many Android
> >> > devices use fbdev now and at least double buffer is needed for these
> >> > Android devices, so that a buffer flip can be operated. It will need
> >> > some time for Android device vendors to abondon legency fbdev. So multi
> >> > buffer for fbdev is needed.
> >>
> >> How exactly do we expect Android to move away from fbdev if we add features to
> >> the fbdev compat layer ? I'd much rather make it clear to them that fbdev is a
> >> thing from the past and that they'd better migrate now.
> >
> > If your point is that merging this patch will slow down the Android
> > move away from fbdev, I disagree with that (obviously).
> >
> > I don't care at all about Android on my platform of choice, but don't
> > see how that merging this patch will change anything.
> >
> > Let's be honest, Android trees typically have thousands of patches on
> > top of mainline. Do you think a simple, 15 LoC, patch will make any
> > difference to vendors? If they want to stay on fbdev and have that
> > feature, they'll just merge this patch, done.
> 
> So, in that case, why not just let them do that? They'd already have
> to add patches to use this, surely; we don't have anything in mainline
> kernels which allows people to actually use this larger allocation.
> Apart from software mmap() and using panning to do flips, but I'm
> taking it as a given that people shipping Android on their devices
> aren't using software rendering.

My point was that you're not doing it more difficult for people not
willing to contribute upstream, you're just making it more difficult
for people who want to contribute.

The whole argument to engage vendors upstream is that we sell them
that eventually they will be able to just use whatever kernel release
is on kernel.org or in their distro of choice.

If those people depend on a feature that is entirely rejected
upstream, then they'll have to carry that patch in their tree,
creating a BSP in the process. And that reduces greatly the strength
of the "you should contribute" argument, making them less involved.

> > However, what I do see is that three different people/organisations
> > have now expressed interest in that feature, on three different
> > SoCs. If that patch needed a significant rework of the fbdev layer,
> > then yes, I might agree that it's not worth it. But in this case, it's
> > pretty trivial.
> >
> > The only people you're "punishing" here with that kind of concern are
> > the people who actually play fair and want not to have any patches and
> > everything upstream.
> 
> I would hazard a guess that most users of this have out-of-tree GPU
> drivers.

Out of tree GPU drivers, that can be distributed separately from the
kernel, just like any out of tree module can. This doesn't require any
kernel patches at all.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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