Frame buffer and plane difference

[Rob Clark]

On Thu, Jun 5, 2014 at 12:49 AM, Anand Sinha <anand.sinha85 at gmail.com> wrote:
> Hi Rob,
>
> Thanks for the answer.so as per the definitions I understand the below
> points.
> a) frame buffer here signifies the scan out buffer which encompasses
> complete display.

originally, before planes, the frame buffer covered the complete
display.  With the advent of planes, now you have crtc scanning fb's
(covering complete display) as well as plane(s) scanning out fb's (not
necessarily covering the complete display).  The more recent
introduction of primary planes (ie. now a crtc doesn't directly scan
out, only planes do.. with some backwards compat stuff to redirect
stuff that used to be in the crtc to the primary plane), even the
primary layer does not have to cover the complete display (if hw
supports that)

> b)when I see the definition of plane in the doc book it says " planes are
> associated with a frame buffer to crop a portion of the image source and
> optionally scale it to destination size". I assume this is not the same
> frame buffer as point a.

well, there is no restriction..  you could of course scanout the same
fb on both a plane and crtc/primary-plane.

The way to summarise it:

framebuffer:  a piece of memory plus some metadata..  typically a GEM
buffer for that piece of memory.  But a GEM buffer does not typically
have a width/height/pixelformat/etc.  That extra metadata are
properties of the fb.

using legacy APIs, you can configure a crtc to scanout an fb to cover
the entire display.

Using the newer more flexible plane APIs, you have more control.  But
you may still have a hw restriction that the primary plane layer must
cover full screen.

BR,
-R

> c)planes are overlayed or blended over the scanout buffer.
>
> Regards,
> Anand
>
> On 23-May-2014 7:25 PM, "Rob Clark" <robdclark at gmail.com> wrote:
>>
>> See:
>>
>>
>> http://people.freedesktop.org/~danvet/drm/drm-mode-setting.html#idp57325824
>>
>> although the docbook probably should be updated (now that we have
>> primary-planes) to read something more like: "...that provide a source
>> of pixels to scanout to a plane"
>>
>> (since a plane is basically the part of display controller block that
>> reads in the pixel data from the fb)
>>
>>
>>
>> On Thu, May 22, 2014 at 6:30 PM, Anand Sinha <anand.sinha85 at gmail.com>
>> wrote:
>> > Hello,
>> >
>> > Can someone help me understand the basic difference between a plane and
>> > a
>> > frame buffer.
>> >
>> > Regards,
>> > Anand
>> >
>> >
>> > _______________________________________________
>> > dri-devel mailing list
>> > dri-devel at lists.freedesktop.org
>> > http://lists.freedesktop.org/mailman/listinfo/dri-devel
>> >