[PATCH 6/8] drm/bochs: phase 3: provide a custom ->atomic_commit implementation

[Pekka Paalanen]

On Thu, 16 Jul 2015 20:20:39 +0800
John Hunter <zhjwpku at gmail.com> wrote:

> From: Zhao Junwang <zhjwpku at gmail.com>
> 
> This supports the asynchronous commits, required for page-flipping
> Since it's virtual hw it's ok to commit async stuff right away, we
> never have to wait for vblank.

Hi,

in theory, yes. This is what a patch to bochs implemented not too long
ago, so AFAIK you are only replicating the existing behaviour.

However, if userspace doing an async commit (or sync, I suppose) does
not incur any waits in the kernel in e.g. sending the page flip event,
then flip driven programs (e.g. a Wayland compositor, say, Weston)
will be running its rendering loop as a busy-loop, because the kernel
does not throttle it to the (virtual) display refresh rate.

This will cause maximal CPU usage and poor user experience as
everything else needs to fight for CPU time and event dispatch to get
through, like input.

I would hope someone could do a follow-up to implement a refresh cycle
emulation based on a clock. Userspace expects page flips to happen at
most at refresh rate when asking for vblank-synced flips. It's only
natural for userspace to drive its rendering loop based on the vblank
cycle.


Thanks,
pq

> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Signed-off-by: Zhao Junwang <zhjwpku at gmail.com>
> ---
>  drivers/gpu/drm/bochs/bochs_mm.c |    9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/bochs/bochs_mm.c b/drivers/gpu/drm/bochs/bochs_mm.c
> index c1d819c..37ac2ca 100644
> --- a/drivers/gpu/drm/bochs/bochs_mm.c
> +++ b/drivers/gpu/drm/bochs/bochs_mm.c
> @@ -545,8 +545,15 @@ bochs_user_framebuffer_create(struct drm_device *dev,
>  	return &bochs_fb->base;
>  }
>  
> +static int bochs_atomic_commit(struct drm_device *dev,
> +			     struct drm_atomic_state *a,
> +			     bool async)
> +{
> +	return drm_atomic_helper_commit(dev, a, false);
> +}
> +
>  const struct drm_mode_config_funcs bochs_mode_funcs = {
>  	.fb_create = bochs_user_framebuffer_create,
>  	.atomic_check = drm_atomic_helper_check,
> -	.atomic_commit = drm_atomic_helper_commit,
> +	.atomic_commit = bochs_atomic_commit,
>  };