[PATCH hwc v1] [RFC] drm_hwcomposer: Flatten composition using writeback connector

[Sean Paul]

On Tue, Mar 20, 2018 at 11:26:39AM +0000, Alexandru-Cosmin Gheorghe wrote:
> Hi Sean,
> 
> Thank you for the feedback.
> I will comeback with v2, later on.
> 
> On Mon, Mar 19, 2018 at 02:03:54PM -0400, Sean Paul wrote:
> > On Tue, Mar 13, 2018 at 04:21:20PM +0000, Alexandru Gheorghe wrote:
> > > This patchset tries to add support for using writeback connector to
> > > flatten a scene when it doesn't change for a while. This idea had
> > > been floated around on IRC here [1] and here [2].
> > > 
> > > Developed on top of the latest writeback series, sent by Liviu here
> > > [3].
> > > 
> > > Probably the patch should/could be broken in more patches, but since I
> > > want to put this out there to get feedback, I kept them as a single
> > > patch for now.
> > 
> > Thank you for your patch, it's looking good. Feel free to submit the v2 in
> > multiple patches. Also note that we've migrated to gitlab, the new tree is
> > located here:
> > 
> > https://gitlab.freedesktop.org/drm-hwcomposer/drm-hwcomposer
> 
> This seems to be behind
> https://anongit.freedesktop.org/git/drm_hwcomposer.git.
> Is this location deprecated or does it serve other purposes.
> 

Ah, thanks for catching this. It's a migration glitch, I've made it whole
again.

<snip />

> > >  #include "drmdisplaycompositor.h"
> > > 
> > >  #include <pthread.h>
> > > @@ -36,9 +35,24 @@
> > >  #include "drmplane.h"
> > >  #include "drmresources.h"
> > >  #include "glworker.h"
> > > +static const uint32_t kWaitWritebackFence = 100; //ms
> > > 
> > >  namespace android {
> > > 
> > > +class CompositorVsyncCallback : public VsyncCallback {
> > > + public:
> > > +  CompositorVsyncCallback(DrmDisplayCompositor *compositor)
> > > +      :compositor_(compositor) {
> > > +  }
> > > +
> > > +  void Callback(int display, int64_t timestamp) {
> > > +    compositor_->Vsync(display, timestamp);
> > > +  }
> > > +
> > > + private:
> > > +  DrmDisplayCompositor *compositor_;
> > > +};
> > > +
> > 
> > The GLCompositor already has flattening, so we should tie into that. I assume
> > writeback is going to be more efficient than using GPU (given that the GPU is
> > probably turned off in these scenarios), so you're probably safe to use
> > writeback when available and fall back to GL if it's not.
> > 
> > 
> 
> As far as I understood, this was done by the drmcompositorworker.cpp
> by calling SquashAll but that's removed from build now. GLCompositor
> seems to do compositing only when there are not enough overlays or 
> atomic check fails.
> 
> Am I missing something ? 
> Is it still doing flattening when then scene does not change ?

https://gitlab.freedesktop.org/drm-hwcomposer/drm-hwcomposer/blob/master/drmcompositorworker.cpp#L62

If the scene doesn't change, everything is squashed.

<snip />

> > > diff --git a/drmresources.cpp b/drmresources.cpp
> > > index 32dd376..880cef2 100644
> > > --- a/drmresources.cpp
> > > +++ b/drmresources.cpp
> > > @@ -33,6 +33,8 @@
> > >  #include <cutils/log.h>
> > >  #include <cutils/properties.h>
> > > 
> > > +#define DRM_CLIENT_CAP_WRITEBACK_CONNECTORS    4
> > > +
> > 
> > Presumably this should come from libdrm headers?
> 
> Yes it will, this was just a quick fix to be able to compile against
> older libdrm.
> 
> Btw, this would make drm_hwcomposer compilable only with newer libdrm
> version, but I suppose that's acceptable.

You can use preprocessor checks to determine if
DRM_CLIENT_CAP_WRITEBACK_CONNECTORS is defined, and disable the functionality if
it's not.

> 
> > 
> > >  namespace android {
> > > 
> > >  DrmResources::DrmResources() : event_listener_(this) {
> > > @@ -65,6 +67,11 @@ int DrmResources::Init() {
> > >      return ret;
> > >    }
> > > 
> > > +  ret = drmSetClientCap(fd(), DRM_CLIENT_CAP_WRITEBACK_CONNECTORS, 1);
> > > +  if (ret) {
> > > +    ALOGI("Failed to set writeback cap %d", ret);
> > > +    ret = 0;
> > > +  }
> > >    drmModeResPtr res = drmModeGetResources(fd());
> > >    if (!res) {
> > >      ALOGE("Failed to get DrmResources resources");
> > > @@ -77,6 +84,7 @@ int DrmResources::Init() {
> > >        std::pair<uint32_t, uint32_t>(res->max_width, res->max_height);
> > > 
> > >    bool found_primary = false;
> > > +  int primary_index = 0;
> > >    int display_num = 1;
> > > 
> > >    for (int i = 0; !ret && i < res->count_crtcs; ++i) {
> > > @@ -162,18 +170,22 @@ int DrmResources::Init() {
> > >      if (conn->internal() && !found_primary) {
> > >        conn->set_display(0);
> > >        found_primary = true;
> > > -    } else {
> > > +    } else if (conn->external()) {
> > > +      if (!found_primary) primary_index++;
> > 
> > This is against style guide (and same below).
> 
> I suppose that's what happens when you skip some steps from 
> "Submitting patches", clang will catch this next time.
> 

I might look into adding a hook to gitlab to automatically run this, time
permitting.

Sean

<snip />
-- 
Sean Paul, Software Engineer, Google / Chromium OS