[PATCH] drm/vkms: Add support for vkms work without vblank

[Rodrigo Siqueira]

On 08/21, Daniel Vetter wrote:
> On Mon, Aug 20, 2018 at 08:55:44PM -0300, Rodrigo Siqueira wrote:
> > Currently, vkms needs VBlank to work well. This patch adds another
> > operation model that make vkms works without VBlank support.
> > 
> > Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo at gmail.com>
> > ---
> >  drivers/gpu/drm/vkms/vkms_crtc.c | 10 ++++++++++
> >  drivers/gpu/drm/vkms/vkms_drv.c  | 12 ++++++++++--
> >  drivers/gpu/drm/vkms/vkms_drv.h  |  1 +
> >  3 files changed, 21 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
> > index bfe6e0312cc4..001fa5c1d326 100644
> > --- a/drivers/gpu/drm/vkms/vkms_crtc.c
> > +++ b/drivers/gpu/drm/vkms/vkms_crtc.c
> > @@ -145,12 +145,22 @@ static const struct drm_crtc_funcs vkms_crtc_funcs = {
> >  static void vkms_crtc_atomic_enable(struct drm_crtc *crtc,
> >  				    struct drm_crtc_state *old_state)
> >  {
> > +	struct vkms_output *vkms_out = drm_crtc_to_vkms_output(crtc);
> > +
> > +	if (vkms_out->disable_vblank)
> > +		return;
> > +
> >  	drm_crtc_vblank_on(crtc);
> >  }
> >  
> >  static void vkms_crtc_atomic_disable(struct drm_crtc *crtc,
> >  				     struct drm_crtc_state *old_state)
> >  {
> > +	struct vkms_output *vkms_out = drm_crtc_to_vkms_output(crtc);
> > +
> > +	if (vkms_out->disable_vblank)
> > +		return;
> > +
> >  	drm_crtc_vblank_off(crtc);
> >  }
> >  
> > diff --git a/drivers/gpu/drm/vkms/vkms_drv.c b/drivers/gpu/drm/vkms/vkms_drv.c
> > index 5d78bd97e69c..f30c4113d4c4 100644
> > --- a/drivers/gpu/drm/vkms/vkms_drv.c
> > +++ b/drivers/gpu/drm/vkms/vkms_drv.c
> > @@ -21,6 +21,10 @@
> >  
> >  static struct vkms_device *vkms_device;
> >  
> > +static bool disable_vblank;
> > +module_param_named(disablevblank, disable_vblank, bool, 0444);
> > +MODULE_PARM_DESC(disablevblank, "Disable vblank interrupt");
> 
> I think the info output is much clearer, I'd rename this to virtual_hw,
> with 0 as default, and a help text along of "Enable virtual HW mode
> (disables vblanks and immediately completes flips)."

You right, virtual_hw is much better :)
I will fix it.
 
> > +
> >  static const struct file_operations vkms_driver_fops = {
> >  	.owner		= THIS_MODULE,
> >  	.open		= drm_open,
> > @@ -105,9 +109,13 @@ static int __init vkms_init(void)
> >  		goto out_fini;
> >  	}
> >  
> > -	vkms_device->drm.irq_enabled = true;
> > +	vkms_device->output.disable_vblank = disable_vblank;
> > +	vkms_device->drm.irq_enabled = !disable_vblank ? true : false;
> > +
> > +	if (disable_vblank)
> > +		DRM_INFO("Virtual Hardware mode enabled");
> >  
> > -	ret = drm_vblank_init(&vkms_device->drm, 1);
> > +	ret = (disable_vblank) ? 0 : drm_vblank_init(&vkms_device->drm, 1);
> >  	if (ret) {
> >  		DRM_ERROR("Failed to vblank\n");
> >  		goto out_fini;
> > diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
> > index f156c930366a..9ee862e6f7e5 100644
> > --- a/drivers/gpu/drm/vkms/vkms_drv.h
> > +++ b/drivers/gpu/drm/vkms/vkms_drv.h
> > @@ -52,6 +52,7 @@ struct vkms_output {
> >  	struct drm_encoder encoder;
> >  	struct drm_connector connector;
> >  	struct hrtimer vblank_hrtimer;
> > +	bool disable_vblank;
> >  	ktime_t period_ns;
> >  	struct drm_pending_vblank_event *event;
> >  	bool crc_enabled;
> 
> The vblank handling parts look correct to me.
> 
> Does kms_flip work with this? Virtual hw immediately completes the
> drm_crtc_state->event in the atomic_update callback, that part seems to be
> missing from your patch.

I tested with patched version of kms_flip to skip vblank. Here is the patch:

https://patchwork.kernel.org/patch/10570861/

About the drm_crtc_state->event, afaiu, the code in the function
vkms_crtc_atomic_flush immediately send the vblank event to the user
space via drm_crtc_send_vblank_event which make things work in the user
space. Make sense? Or I missed something?

> Also I like the idea of switching away from the generic -EINVAL to -ENOTTY
> for when vblanks aren't supported. Do you have that somewhere (as a
> separate patch).

I already started to investigate if the change between -EINVAL to
-ENOTTY will break an existing kms userspace compositors. I already
checked:

* kwin
* sway
* wayland-core
* weston

I still have to check the X drivers, chromium, and drm_hwc.

Thanks for the feedback.

> -Daniel
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Rodrigo Siqueira
http://siqueira.tech
Graduate Student
Department of Computer Science
University of São Paulo