[PATCH RFC 2/8] DRM: Armada: Add Armada DRM driver

[Russell King - ARM Linux]

On Tue, Jun 11, 2013 at 12:01:59AM +0200, Daniel Vetter wrote:
> On Mon, Jun 10, 2013 at 9:59 PM, Rob Clark <robdclark at gmail.com> wrote:
> > On Mon, Jun 10, 2013 at 1:06 PM, Russell King - ARM Linux
> > <linux at arm.linux.org.uk> wrote:
> >> On Mon, Jun 10, 2013 at 11:57:32AM -0400, Rob Clark wrote:
> >>> On Sun, Jun 9, 2013 at 3:29 PM, Russell King
> >>> <rmk+kernel at arm.linux.org.uk> wrote:
> >>> > This patch adds support for the pair of LCD controllers on the Marvell
> >>> > Armada 510 SoCs.  This driver supports:
> >>> > - multiple contiguous scanout buffers for video and graphics
> >>> > - shm backed cacheable buffer objects for X pixmaps for Vivante GPU
> >>> >   acceleration
> >>> > - dual lcd0 and lcd1 crt operation
> >>> > - video overlay on each LCD crt
> >>>
> >>> Any particular reason for not exposing the overlays as drm_plane's?
> >>> That is probably something that should change before merging the
> >>> driver.
> >>
> >> Only through not understanding much of DRM when I started this.
> >> Provided DRM planes can do everything that I'm already doing with
> >> the overlay support, then yes.  Otherwise, I want to stick with this
> >> which is modelled after the i915 overlay interface.
> 
> Oh i915 overlays aren't a good reason ;-) I've done those way back
> when drm didn't have any plane infrastructure and pretty much as my
> very contribution. So totally lacked any clue.
> 
> If I can scrap together a bit of time I want to port the legacy
> overlay code over to drm planes (and remap the current ioctl interface
> to the drm plane interface for backwards compat). But atm that's
> crushed under a giant pile of other todo items.

Aren't we all :(  The amount of time I have to touch this has reduced
substantially over the last couple of months, which is why there's been
a few weeks between the RFC's for this.

The issue here is that with the overlay interface, all I have to do
with one of these pass-by-phys-address things which the X server gets
is to:

1. associate the phys address with a gem object
2. pass it in via the overlay interface

With the DRM plane stuff, this gets more complicated:

1. associate the phys address with a gem object
2. call another driver private ioctl to bind the gem object to a framebuffer
   (there is no support for this in the generic ioctl API afaics) which
   has to allocate and setup a framebuffer
3. use setplane to update the plane

That all increases the expense of overlay, and adds further memory
allocations to the path (and frees when the previous framebuffer is
discarded.)

That all makes for higher load due to more CPU expense.