Re: [Intel-gfx] [PATCH 3/4] drm/i915: Use vblank workers for gamma updates

Shankar, Uma
2021-11-08T20:41:31Z

> -----Original Message-----
> From: Intel-gfx <intel-gfx-bounces@lists.freedesktop.org> On Behalf Of Ville Syrjala
> Sent: Thursday, October 21, 2021 4:04 AM
> To: intel-gfx@lists.freedesktop.org
> Cc: Lyude Paul <lyude@redhat.com>
> Subject: [Intel-gfx] [PATCH 3/4] drm/i915: Use vblank workers for gamma updates
>
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
>
> The pipe gamma registers are single buffered so they should only be updated during
> the vblank to avoid screen tearing. In fact they really should only be updated
> between start of vblank and frame start because that is the only time the pipe is
> guaranteed to be empty. Already at frame start the pipe begins to fill up with data
> for the next frame.
>
> Unfortunately frame start happens ~1 scanline after the start of vblank which in
> practice doesn't always leave us enough time to finish the gamma update in time
> (gamma LUTs can be several KiB of data we have to bash into the registers).
> However we must try our best and so we'll add a vblank work for each pipe from
> where we can do the gamma update. Additionally we could consider pushing frame
> start forward to the max of ~4 scanlines after start of vblank. But not sure that's
> exactly a validated configuration.
> As it stands the ~100 first pixels tend to make it through with the old gamma values.
>
> Even though the vblank worker is running on a high prority thread we still have to
> contend with C-states. If the CPU happens be in a deep C-state when the vblank
> interrupt arrives even the irq handler gets delayed massively (I've observed dozens of
> scanlines worth of latency). To avoid that problem we'll use the qos mechanism to
> keep the CPU awake while the vblank work is scheduled.
>
> With all this hooked up we can finally enjoy near atomic gamma updates. It even
> works across several pipes from the same atomic commit which previously was a
> total fail because we did the gamma updates for each pipe serially after waiting for
> all pipes to have latched the double buffered registers.
>
> In the future the DSB should take over this responsibility which will hopefully avoid
> some of these issues.
>
> Kudos to Lyude for finishing the actual vblank workers.
> Works like the proverbial train toilet.
>
> v2: Add missing intel_atomic_state fwd declaration
> v3: Clean up properly when not scheduling the worker
> v4: Clean up the rest and add tracepoints
>
> CC: Lyude Paul <lyude@redhat.com>
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
> drivers/gpu/drm/i915/display/intel_crtc.c | 76 ++++++++++++++++++-
> drivers/gpu/drm/i915/display/intel_crtc.h | 4 +-
> drivers/gpu/drm/i915/display/intel_display.c | 9 +--
> .../drm/i915/display/intel_display_types.h | 8 ++
> drivers/gpu/drm/i915/i915_trace.h | 42 ++++++++++
> 5 files changed, 129 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_crtc.c
> b/drivers/gpu/drm/i915/display/intel_crtc.c
> index 0f8b48b6911c..4758c61adae8 100644
> --- a/drivers/gpu/drm/i915/display/intel_crtc.c
> +++ b/drivers/gpu/drm/i915/display/intel_crtc.c
> @@ -3,12 +3,14 @@
> * Copyright © 2020 Intel Corporation
> */
> #include <linux/kernel.h>
> +#include <linux/pm_qos.h>
> #include <linux/slab.h>
>
> #include <drm/drm_atomic_helper.h>
> #include <drm/drm_fourcc.h>
> #include <drm/drm_plane.h>
> #include <drm/drm_plane_helper.h>
> +#include <drm/drm_vblank_work.h>
>
> #include "i915_trace.h"
> #include "i915_vgpu.h"
> @@ -167,6 +169,8 @@ static void intel_crtc_destroy(struct drm_crtc *_crtc) {
> struct intel_crtc *crtc = to_intel_crtc(_crtc);
>
> + cpu_latency_qos_remove_request(&crtc->vblank_pm_qos);
> +
> drm_crtc_cleanup(&crtc->base);
> kfree(crtc);
> }
> @@ -344,6 +348,8 @@ int intel_crtc_init(struct drm_i915_private *dev_priv, enum
> pipe pipe)
>
> intel_crtc_crc_init(crtc);
>
> + cpu_latency_qos_add_request(&crtc->vblank_pm_qos,
> +PM_QOS_DEFAULT_VALUE);
> +
> drm_WARN_ON(&dev_priv->drm, drm_crtc_index(&crtc->base) != crtc-
> >pipe);
>
> return 0;
> @@ -354,6 +360,65 @@ int intel_crtc_init(struct drm_i915_private *dev_priv, enum
> pipe pipe)
> return ret;
> }
>
> +static bool intel_crtc_needs_vblank_work(const struct intel_crtc_state
> +*crtc_state) {
> + return crtc_state->hw.active &&
> + !intel_crtc_needs_modeset(crtc_state) &&
> + !crtc_state->preload_luts &&
> + (crtc_state->uapi.color_mgmt_changed ||
> + crtc_state->update_pipe);
> +}
> +
> +static void intel_crtc_vblank_work(struct kthread_work *base) {
> + struct drm_vblank_work *work = to_drm_vblank_work(base);
> + struct intel_crtc_state *crtc_state =
> + container_of(work, typeof(*crtc_state), vblank_work);
> + struct intel_crtc *crtc = to_intel_crtc(crtc_state->uapi.crtc);
> +
> + trace_intel_crtc_vblank_work_start(crtc);
> +
> + intel_color_load_luts(crtc_state);
> +
> + if (crtc_state->uapi.event) {
> + spin_lock_irq(&crtc->base.dev->event_lock);
> + drm_crtc_send_vblank_event(&crtc->base, crtc_state->uapi.event);
> + crtc_state->uapi.event = NULL;
> + spin_unlock_irq(&crtc->base.dev->event_lock);
> + }
> +
> + trace_intel_crtc_vblank_work_end(crtc);
> +}
> +
> +static void intel_crtc_vblank_work_init(struct intel_crtc_state
> +*crtc_state) {
> + struct intel_crtc *crtc = to_intel_crtc(crtc_state->uapi.crtc);
> +
> + drm_vblank_work_init(&crtc_state->vblank_work, &crtc->base,
> + intel_crtc_vblank_work);
> + /*
> + * Interrupt latency is critical for getting the vblank
> + * work executed as early as possible during the vblank.
> + */
> + cpu_latency_qos_update_request(&crtc->vblank_pm_qos, 0); }
> +
> +void intel_wait_for_vblank_works(struct intel_atomic_state *state) {
> + struct intel_crtc_state *crtc_state;
> + struct intel_crtc *crtc;
> + int i;
> +
> + for_each_new_intel_crtc_in_state(state, crtc, crtc_state, i) {
> + if (!intel_crtc_needs_vblank_work(crtc_state))
> + continue;
> +
> + drm_vblank_work_flush(&crtc_state->vblank_work);
> + cpu_latency_qos_update_request(&crtc->vblank_pm_qos,
> + PM_QOS_DEFAULT_VALUE);
> + }
> +}
> +
> int intel_usecs_to_scanlines(const struct drm_display_mode *adjusted_mode,
> int usecs)
> {
> @@ -387,7 +452,7 @@ static int intel_mode_vblank_start(const struct
> drm_display_mode *mode)
> * until a subsequent call to intel_pipe_update_end(). That is done to
> * avoid random delays.
> */
> -void intel_pipe_update_start(const struct intel_crtc_state *new_crtc_state)
> +void intel_pipe_update_start(struct intel_crtc_state *new_crtc_state)
> {
> struct intel_crtc *crtc = to_intel_crtc(new_crtc_state->uapi.crtc);
> struct drm_i915_private *dev_priv = to_i915(crtc->base.dev); @@ -402,6
> +467,9 @@ void intel_pipe_update_start(const struct intel_crtc_state
> *new_crtc_state)
> if (new_crtc_state->uapi.async_flip)
> return;
>
> + if (intel_crtc_needs_vblank_work(new_crtc_state))
> + intel_crtc_vblank_work_init(new_crtc_state);
> +
> if (new_crtc_state->vrr.enable)
> vblank_start = intel_vrr_vmax_vblank_start(new_crtc_state);
> else
> @@ -557,7 +625,11 @@ void intel_pipe_update_end(struct intel_crtc_state
> *new_crtc_state)
> * Would be slightly nice to just grab the vblank count and arm the
> * event outside of the critical section - the spinlock might spin for a
> * while ... */
> - if (new_crtc_state->uapi.event) {
> + if (intel_crtc_needs_vblank_work(new_crtc_state)) {
> + drm_vblank_work_schedule(&new_crtc_state->vblank_work,
> + drm_crtc_accurate_vblank_count(&crtc-
> >base) + 1,
> + false);
> + } else if (new_crtc_state->uapi.event) {
> drm_WARN_ON(&dev_priv->drm,
> drm_crtc_vblank_get(&crtc->base) != 0);
>
> diff --git a/drivers/gpu/drm/i915/display/intel_crtc.h
> b/drivers/gpu/drm/i915/display/intel_crtc.h
> index 22363fbbc925..25eb58bce0dd 100644
> --- a/drivers/gpu/drm/i915/display/intel_crtc.h
> +++ b/drivers/gpu/drm/i915/display/intel_crtc.h
> @@ -11,6 +11,7 @@
> enum pipe;
> struct drm_display_mode;
> struct drm_i915_private;
> +struct intel_atomic_state;
> struct intel_crtc;
> struct intel_crtc_state;
>
> @@ -24,7 +25,8 @@ void intel_crtc_state_reset(struct intel_crtc_state *crtc_state,
> u32 intel_crtc_get_vblank_counter(struct intel_crtc *crtc); void
> intel_crtc_vblank_on(const struct intel_crtc_state *crtc_state); void
> intel_crtc_vblank_off(const struct intel_crtc_state *crtc_state); -void
> intel_pipe_update_start(const struct intel_crtc_state *new_crtc_state);
> +void intel_pipe_update_start(struct intel_crtc_state *new_crtc_state);
> void intel_pipe_update_end(struct intel_crtc_state *new_crtc_state);
> +void intel_wait_for_vblank_works(struct intel_atomic_state *state);
>
> #endif
> diff --git a/drivers/gpu/drm/i915/display/intel_display.c
> b/drivers/gpu/drm/i915/display/intel_display.c
> index 79a7552af7b5..1375d963c0a8 100644
> --- a/drivers/gpu/drm/i915/display/intel_display.c
> +++ b/drivers/gpu/drm/i915/display/intel_display.c
> @@ -8818,6 +8818,8 @@ static void intel_atomic_commit_tail(struct
> intel_atomic_state *state)
> intel_set_cdclk_post_plane_update(state);
> }
>
> + intel_wait_for_vblank_works(state);

Change looks perfect to me. Maybe we can just change this s/works/workers.
Overall a nice idea.

Reviewed-by: Uma Shankar <uma.shankar@intel.com>

> +
> /* FIXME: We should call drm_atomic_helper_commit_hw_done() here
> * already, but still need the state for the delayed optimization. To
> * fix this:
> @@ -8832,13 +8834,6 @@ static void intel_atomic_commit_tail(struct
> intel_atomic_state *state)
> for_each_new_intel_crtc_in_state(state, crtc, new_crtc_state, i) {
> if (new_crtc_state->uapi.async_flip)
> intel_crtc_disable_flip_done(state, crtc);
> -
> - if (new_crtc_state->hw.active &&
> - !intel_crtc_needs_modeset(new_crtc_state) &&
> - !new_crtc_state->preload_luts &&
> - (new_crtc_state->uapi.color_mgmt_changed ||
> - new_crtc_state->update_pipe))
> - intel_color_load_luts(new_crtc_state);
> }
>
> /*
> diff --git a/drivers/gpu/drm/i915/display/intel_display_types.h
> b/drivers/gpu/drm/i915/display/intel_display_types.h
> index 1e42bf901263..8bb9264022f5 100644
> --- a/drivers/gpu/drm/i915/display/intel_display_types.h
> +++ b/drivers/gpu/drm/i915/display/intel_display_types.h
> @@ -28,6 +28,7 @@
>
> #include <linux/async.h>
> #include <linux/i2c.h>
> +#include <linux/pm_qos.h>
> #include <linux/pwm.h>
> #include <linux/sched/clock.h>
>
> @@ -41,6 +42,7 @@
> #include <drm/drm_probe_helper.h>
> #include <drm/drm_rect.h>
> #include <drm/drm_vblank.h>
> +#include <drm/drm_vblank_work.h>
> #include <drm/i915_mei_hdcp_interface.h> #include <media/cec-notifier.h>
>
> @@ -1241,6 +1243,9 @@ struct intel_crtc_state {
> u8 link_count;
> u8 pixel_overlap;
> } splitter;
> +
> + /* for loading single buffered registers during vblank */
> + struct drm_vblank_work vblank_work;
> };
>
> enum intel_pipe_crc_source {
> @@ -1325,6 +1330,9 @@ struct intel_crtc {
> /* scalers available on this crtc */
> int num_scalers;
>
> + /* for loading single buffered registers during vblank */
> + struct pm_qos_request vblank_pm_qos;
> +
> #ifdef CONFIG_DEBUG_FS
> struct intel_pipe_crc pipe_crc;
> #endif
> diff --git a/drivers/gpu/drm/i915/i915_trace.h b/drivers/gpu/drm/i915/i915_trace.h
> index 9795f456cccf..fc48a16d7a4f 100644
> --- a/drivers/gpu/drm/i915/i915_trace.h
> +++ b/drivers/gpu/drm/i915/i915_trace.h
> @@ -404,6 +404,48 @@ TRACE_EVENT(intel_fbc_nuke,
>
> /* pipe updates */
>
> +TRACE_EVENT(intel_crtc_vblank_work_start,
> + TP_PROTO(struct intel_crtc *crtc),
> + TP_ARGS(crtc),
> +
> + TP_STRUCT__entry(
> + __field(enum pipe, pipe)
> + __field(u32, frame)
> + __field(u32, scanline)
> + ),
> +
> + TP_fast_assign(
> + __entry->pipe = crtc->pipe;
> + __entry->frame = intel_crtc_get_vblank_counter(crtc);
> + __entry->scanline = intel_get_crtc_scanline(crtc);
> + ),
> +
> + TP_printk("pipe %c, frame=%u, scanline=%u",
> + pipe_name(__entry->pipe), __entry->frame,
> + __entry->scanline)
> +);
> +
> +TRACE_EVENT(intel_crtc_vblank_work_end,
> + TP_PROTO(struct intel_crtc *crtc),
> + TP_ARGS(crtc),
> +
> + TP_STRUCT__entry(
> + __field(enum pipe, pipe)
> + __field(u32, frame)
> + __field(u32, scanline)
> + ),
> +
> + TP_fast_assign(
> + __entry->pipe = crtc->pipe;
> + __entry->frame = intel_crtc_get_vblank_counter(crtc);
> + __entry->scanline = intel_get_crtc_scanline(crtc);
> + ),
> +
> + TP_printk("pipe %c, frame=%u, scanline=%u",
> + pipe_name(__entry->pipe), __entry->frame,
> + __entry->scanline)
> +);
> +
> TRACE_EVENT(intel_pipe_update_start,
> TP_PROTO(struct intel_crtc *crtc),
> TP_ARGS(crtc),
> --
> 2.32.0