Re: [PATCH V2] drm/vkms: Add vblank events simulated by hrtimers

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Jul 05, 2018 at 12:48:43AM -0300, Rodrigo Siqueira wrote:
> This commit adds regular vblank events simulated through hrtimers, which
> is a feature required by VKMS to mimic real hardware. Notice that
> keeping the periodicity as close as possible to the one specified in
> user space represents a vital constraint. In this sense, the callback
> function implements an algorithm based on module operations that avoids
> the accumulation of errors.
> 
> Assuming we begin operating at timestamp 0, every event should happen at
> timestamps that are multiples of the requested period, i.e.,
> current_timestamp % period == 0. Since we do *not* generally start at
> timestamp 0, we calculate the modulus of this initial timestamp and try
> to make all following events happen when current_timestamp % period ==
> initial_timestamp % period (variables “current_offset” and
> “base_offset”). We do this by measuring the difference between them at
> each iteration and adjusting the next waiting period accordingly.
> 
> Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>

Looks good. I've done a detailed review and found a bunch of things to
polish.

When resending patches please include a short summary of the changes
you've done. This helps reviewers a lot. E.g. for this one here:

Changes since v1:
- Compute the timer interval using the mode.

For the next revision you keep that and add

Changes since v2:
- Use the timing information already computed by core code.
- Use absolute timer mode
- and-so-on ...

Cheers, Daniel

> ---
>  drivers/gpu/drm/vkms/vkms_crtc.c | 99 +++++++++++++++++++++++++++++++-
>  drivers/gpu/drm/vkms/vkms_drv.c  |  9 +++
>  drivers/gpu/drm/vkms/vkms_drv.h  | 20 +++++++
>  3 files changed, 126 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
> index 84cc05506b09..35d39d25df34 100644
> --- a/drivers/gpu/drm/vkms/vkms_crtc.c
> +++ b/drivers/gpu/drm/vkms/vkms_crtc.c
> @@ -10,6 +10,83 @@
>  #include <drm/drm_atomic_helper.h>
>  #include <drm/drm_crtc_helper.h>
>  
> +static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
> +{
> +	struct vkms_output *output = container_of(timer, struct vkms_output,
> +						  vblank_hrtimer);
> +	struct drm_crtc *crtc = &output->crtc;
> +	ktime_t new_period, current_timestamp, diff_times, current_offset,
> +		candidate_expires;
> +	unsigned long flags;
> +	int ret_overrun;
> +	bool ret;
> +
> +	current_timestamp = ktime_get();
> +	current_offset = current_timestamp % output->period_ns;
> +	diff_times = ktime_sub(current_offset, output->base_offset);
> +	candidate_expires = ktime_sub(current_timestamp, diff_times);
> +	if (candidate_expires > output->expires)
> +		output->expires = candidate_expires;
> +
> +	ret = drm_crtc_handle_vblank(crtc);
> +	if (!ret)
> +		DRM_ERROR("vkms failure on handling vblank");
> +
> +	if (output->event) {

You're accessing output->event without holding locks, that's not ok. But
see below, you can remove all that code.

> +		spin_lock_irqsave(&crtc->dev->event_lock, flags);
> +		drm_crtc_send_vblank_event(crtc, output->event);
> +		output->event = NULL;
> +		spin_unlock_irqrestore(&crtc->dev->event_lock, flags);
> +		drm_crtc_vblank_put(crtc);
> +	}
> +
> +	current_timestamp = ktime_get();
> +	current_offset = current_timestamp % output->period_ns;
> +	diff_times = ktime_sub(output->base_offset, current_offset);
> +	new_period = ktime_add(output->period_ns, diff_times);
> +	output->expires = ktime_add(current_timestamp, new_period);
> +	ret_overrun = hrtimer_forward_now(&output->vblank_hrtimer, new_period);
> +
> +	return HRTIMER_RESTART;
> +}
> +
> +static int vkms_enable_vblank(struct drm_crtc *crtc)
> +{
> +	struct vkms_output *out = drm_crtc_to_vkms_output(crtc);
> +	unsigned long period = 1000 * crtc->mode.htotal * crtc->mode.vtotal /
> +			       crtc->mode.clock;

General note: Diving stuff in the kernel isn't easy, you need to use
division macros otherwise your code won't build on 32bit.

Wrt the calculation itself: That's already done for you in
drm_calc_timestamping_constants(), no need to reinvent that wheel.

> +	ktime_t current_timestamp;
> +
> +	hrtimer_init(&out->vblank_hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);

Can't we use absolute timer mode here? That avoids all the timestampt
computations.

> +	out->vblank_hrtimer.function = &vkms_vblank_simulate;
> +	out->period_ns = ktime_set(0, period * US_TO_NS);
> +	current_timestamp = ktime_get();
> +	out->base_offset = current_timestamp % out->period_ns;
> +	out->expires = ktime_add(current_timestamp, out->period_ns);
> +	hrtimer_start(&out->vblank_hrtimer, out->period_ns, HRTIMER_MODE_REL);
> +
> +	return 0;
> +}
> +
> +static void vkms_disable_vblank(struct drm_crtc *crtc)
> +{
> +	struct vkms_output *out = drm_crtc_to_vkms_output(crtc);
> +
> +	hrtimer_cancel(&out->vblank_hrtimer);
> +}
> +
> +bool vkms_get_vblank_timestamp(struct drm_device *dev, unsigned int pipe,
> +			       int *max_error, ktime_t *vblank_time,
> +			       bool in_vblank_irq)
> +{
> +	struct vkms_device *vkmsdev = drm_device_to_vkms_device(dev);
> +	struct vkms_output *output = &vkmsdev->output;
> +
> +	*vblank_time = output->expires;
> +
> +	return true;
> +}
> +
>  static const struct drm_crtc_funcs vkms_crtc_funcs = {
>  	.set_config             = drm_atomic_helper_set_config,
>  	.destroy                = drm_crtc_cleanup,
> @@ -17,6 +94,8 @@ static const struct drm_crtc_funcs vkms_crtc_funcs = {
>  	.reset                  = drm_atomic_helper_crtc_reset,
>  	.atomic_duplicate_state = drm_atomic_helper_crtc_duplicate_state,
>  	.atomic_destroy_state   = drm_atomic_helper_crtc_destroy_state,
> +	.enable_vblank		= vkms_enable_vblank,
> +	.disable_vblank		= vkms_disable_vblank,
>  };
>  
>  static int vkms_crtc_atomic_check(struct drm_crtc *crtc,
> @@ -28,11 +107,27 @@ static int vkms_crtc_atomic_check(struct drm_crtc *crtc,
>  static void vkms_crtc_atomic_enable(struct drm_crtc *crtc,
>  				    struct drm_crtc_state *old_state)
>  {
> +	drm_crtc_vblank_on(crtc);

drm_crtc_vblank_off is missing. That should result in some complaints when
you disable a crtc again ... Have you not seen any backtraces in dmesg?

> +}
> +
> +static void vkms_crtc_atomic_begin(struct drm_crtc *crtc,
> +				   struct drm_crtc_state *old_s)
> +{
> +	struct vkms_output *vkms_output = drm_crtc_to_vkms_output(crtc);
> +
> +	if (crtc->state->event) {
> +		crtc->state->event->pipe = drm_crtc_index(crtc);
> +		WARN_ON(drm_crtc_vblank_get(crtc) != 0);
> +
> +		vkms_output->event = crtc->state->event;

You're missing locking here for output->event. Also I expect that you're
hitting the WARN_ON above (or you would, if you would call
drm_crtc_vblank_off).

Much simpler implementation is if you use drm_crtc_arm_vblank_event, plus
the fallback to sending the event out directly if the crtc is disabled.
See tegra_crtc_atomic_begin() for an example.

Also for the long-term plans we have with vkms with the crc stuff it's
better to put this all into the atomic_flush callback, not the
atomic_begin callback.

> +		crtc->state->event = NULL;
> +	}
>  }
>  
>  static const struct drm_crtc_helper_funcs vkms_crtc_helper_funcs = {
> -	.atomic_check  = vkms_crtc_atomic_check,
> -	.atomic_enable = vkms_crtc_atomic_enable,
> +	.atomic_check	= vkms_crtc_atomic_check,
> +	.atomic_enable	= vkms_crtc_atomic_enable,
> +	.atomic_begin	= vkms_crtc_atomic_begin,
>  };
>  
>  int vkms_crtc_init(struct drm_device *dev, struct drm_crtc *crtc,
> diff --git a/drivers/gpu/drm/vkms/vkms_drv.c b/drivers/gpu/drm/vkms/vkms_drv.c
> index fe93f8c17997..6368048c6e54 100644
> --- a/drivers/gpu/drm/vkms/vkms_drv.c
> +++ b/drivers/gpu/drm/vkms/vkms_drv.c
> @@ -55,6 +55,7 @@ static struct drm_driver vkms_driver = {
>  	.dumb_create		= vkms_dumb_create,
>  	.dumb_map_offset	= vkms_dumb_map,
>  	.gem_vm_ops		= &vkms_gem_vm_ops,
> +	.get_vblank_timestamp	= vkms_get_vblank_timestamp,

Ah, the last legacy hook where we don't yet have an option to use
drm_crtc_funcs instead. For now this is good here, but I think it'd be
great to do a follow-up task to clean this up, like we've already done
with the enable/disable_vblank hooks.
>  
>  	.name			= DRIVER_NAME,
>  	.desc			= DRIVER_DESC,
> @@ -102,6 +103,14 @@ static int __init vkms_init(void)
>  		goto out_fini;
>  	}
>  
> +	vkms_device->drm.irq_enabled = true;
> +
> +	ret = drm_vblank_init(&vkms_device->drm, 1);
> +	if (ret) {
> +		DRM_ERROR("Failed to vblank\n");
> +		goto out_fini;
> +	}
> +
>  	ret = vkms_modeset_init(vkms_device);
>  	if (ret)
>  		goto out_unregister;
> diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
> index 76f1720f81a5..b69a223bdfe6 100644
> --- a/drivers/gpu/drm/vkms/vkms_drv.h
> +++ b/drivers/gpu/drm/vkms/vkms_drv.h
> @@ -5,6 +5,7 @@
>  #include <drm/drm.h>
>  #include <drm/drm_gem.h>
>  #include <drm/drm_encoder.h>
> +#include <linux/hrtimer.h>
>  
>  #define XRES_MIN    32
>  #define YRES_MIN    32
> @@ -15,6 +16,9 @@
>  #define XRES_MAX  8192
>  #define YRES_MAX  8192
>  
> +#define DEFAULT_VBLANK_NS 16666666
> +#define US_TO_NS 1000
> +
>  static const u32 vkms_formats[] = {
>  	DRM_FORMAT_XRGB8888,
>  };
> @@ -23,6 +27,11 @@ struct vkms_output {
>  	struct drm_crtc crtc;
>  	struct drm_encoder encoder;
>  	struct drm_connector connector;
> +	struct hrtimer vblank_hrtimer;
> +	ktime_t period_ns;
> +	ktime_t base_offset;
> +	ktime_t expires;
> +	struct drm_pending_vblank_event *event;
>  };
>  
>  struct vkms_device {
> @@ -37,9 +46,20 @@ struct vkms_gem_object {
>  	struct page **pages;
>  };
>  
> +#define drm_crtc_to_vkms_output(target) \
> +	container_of(target, struct vkms_output, crtc)
> +
> +#define drm_device_to_vkms_device(target) \
> +	container_of(target, struct vkms_device, drm)
> +
> +/* CRTC */
>  int vkms_crtc_init(struct drm_device *dev, struct drm_crtc *crtc,
>  		   struct drm_plane *primary, struct drm_plane *cursor);
>  
> +bool vkms_get_vblank_timestamp(struct drm_device *dev, unsigned int pipe,
> +			       int *max_error, ktime_t *vblank_time,
> +			       bool in_vblank_irq);
> +
>  int vkms_output_init(struct vkms_device *vkmsdev);
>  
>  struct drm_plane *vkms_plane_init(struct vkms_device *vkmsdev);
> -- 
> 2.18.0
> 

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