[PATCH v4] drm/i915/display: Optimize panel power-on wait time
Jani Nikula
jani.nikula at intel.com
Tue Jul 29 08:17:48 UTC 2025
On Mon, 28 Jul 2025, Dibin Moolakadan Subrahmanian <dibin.moolakadan.subrahmanian at intel.com> wrote:
> The current wait_panel_status() uses intel_de_wait() with a long timeout
> (5000ms), which is suboptimal on Xe platforms where the underlying
> xe_mmio_wait32() employs an exponential backoff strategy. This leads
> to unnecessary delays during resume or power-on when the panel becomes
> ready earlier than the full timeout.
It's not about the timeout, it's about the exponentially increasing poll
delay.
> This patch replaces intel_de_wait() with read_poll_timeout() +
> intel_de_read() to actively poll the register at given interval and exit
> early when panel is ready, improving resume latency
Please do not say "this patch" in commit messages. Just use the
imperative "Replace ...".
The commit messages is unnecessarily indented with a space.
> Changes in v2:
> Replaced two-phase intel_de_wait() with read_poll_timeout()
> + intel_de_read()
>
> Changes in v3:
> - Add poll_interval_ms argument 'wait_panel_status' function.
> - Modify 'wait_panel_status' callers with proper poll interval
>
> Changes in v4:
> - Change 'wait_panel_off' poll interval to 10ms
>
> Signed-off-by: Dibin Moolakadan Subrahmanian <dibin.moolakadan.subrahmanian at intel.com>
> ---
> drivers/gpu/drm/i915/display/intel_pps.c | 41 +++++++++++++++++-------
> 1 file changed, 30 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_pps.c b/drivers/gpu/drm/i915/display/intel_pps.c
> index b64d0b30f5b1..56ef835fc2eb 100644
> --- a/drivers/gpu/drm/i915/display/intel_pps.c
> +++ b/drivers/gpu/drm/i915/display/intel_pps.c
> @@ -22,6 +22,7 @@
> #include "intel_pps.h"
> #include "intel_pps_regs.h"
> #include "intel_quirks.h"
> +#include <linux/iopoll.h>
Please look at how includes are ordered in every single file in i915.
> static void vlv_steal_power_sequencer(struct intel_display *display,
> enum pipe pipe);
> @@ -600,14 +601,18 @@ void intel_pps_check_power_unlocked(struct intel_dp *intel_dp)
> #define IDLE_CYCLE_MASK (PP_ON | PP_SEQUENCE_MASK | PP_CYCLE_DELAY_ACTIVE | PP_SEQUENCE_STATE_MASK)
> #define IDLE_CYCLE_VALUE (0 | PP_SEQUENCE_NONE | 0 | PP_SEQUENCE_STATE_OFF_IDLE)
>
> +#define PANEL_MAXIMUM_ON_TIME_MS (5000)
The name of the macro is misleading. For single-use things, maybe better
to just keep the value inline as it were.
Side note, the parenthesis are superfluous here.
> +
> static void intel_pps_verify_state(struct intel_dp *intel_dp);
>
> static void wait_panel_status(struct intel_dp *intel_dp,
> - u32 mask, u32 value)
> + u32 mask, u32 value, int poll_interval_ms)
Can we not add the extra parameter please? Can we have a meaningful
default instead? 10 ms? Is the 1 ms poll interval really required?
> {
> struct intel_display *display = to_intel_display(intel_dp);
> struct intel_digital_port *dig_port = dp_to_dig_port(intel_dp);
> i915_reg_t pp_stat_reg, pp_ctrl_reg;
> + int ret;
> + u32 reg_val;
Nitpick, usually just "val".
> lockdep_assert_held(&display->pps.mutex);
>
> @@ -624,14 +629,27 @@ static void wait_panel_status(struct intel_dp *intel_dp,
> intel_de_read(display, pp_stat_reg),
> intel_de_read(display, pp_ctrl_reg));
>
> - if (intel_de_wait(display, pp_stat_reg, mask, value, 5000))
> - drm_err(display->drm,
> - "[ENCODER:%d:%s] %s panel status timeout: PP_STATUS: 0x%08x PP_CONTROL: 0x%08x\n",
> - dig_port->base.base.base.id, dig_port->base.base.name,
> - pps_name(intel_dp),
> - intel_de_read(display, pp_stat_reg),
> - intel_de_read(display, pp_ctrl_reg));
> + if (poll_interval_ms <= 0)
> + poll_interval_ms = 1; //if <0 is passed go with 1ms
Without the parameter, we could get rid of checks like this.
The comment just duplicates what the code already says.
Also, we don't use // comments.
> +
> + ret = read_poll_timeout(intel_de_read, reg_val,
> + ((reg_val & mask) == value),
> + (poll_interval_ms * 1000), // poll intervell
> + (PANEL_MAXIMUM_ON_TIME_MS * 1000), // total timeout (us)
> + true,
> + display, pp_stat_reg);
The outer parenthesis in the parameters are superfluous.
The comments are useless (and have a typo too).
> +
> + if (ret == 0)
> + goto panel_wait_complete;
We do use goto in kernel, but primarily for error handling. Please use
if (ret)
here, and the whole drm_err() thing remains unchanged, and doesn't
become part of the patch...
>
> + drm_err(display->drm,
> + "dibin [ENCODER:%d:%s] %s panel status timeout: PP_STATUS: 0x%08x PP_CONTROL: 0x%08x\n",
...and it'll be easier to notice you've left your name in the debug
logs. Oops.
> + dig_port->base.base.base.id, dig_port->base.base.name,
> + pps_name(intel_dp),
> + intel_de_read(display, pp_stat_reg),
> + intel_de_read(display, pp_ctrl_reg));
> +
> +panel_wait_complete:
> drm_dbg_kms(display->drm, "Wait complete\n");
> }
>
> @@ -644,7 +662,8 @@ static void wait_panel_on(struct intel_dp *intel_dp)
> "[ENCODER:%d:%s] %s wait for panel power on\n",
> dig_port->base.base.base.id, dig_port->base.base.name,
> pps_name(intel_dp));
> - wait_panel_status(intel_dp, IDLE_ON_MASK, IDLE_ON_VALUE);
> +
> + wait_panel_status(intel_dp, IDLE_ON_MASK, IDLE_ON_VALUE, 20);
> }
>
> static void wait_panel_off(struct intel_dp *intel_dp)
> @@ -656,7 +675,7 @@ static void wait_panel_off(struct intel_dp *intel_dp)
> "[ENCODER:%d:%s] %s wait for panel power off time\n",
> dig_port->base.base.base.id, dig_port->base.base.name,
> pps_name(intel_dp));
> - wait_panel_status(intel_dp, IDLE_OFF_MASK, IDLE_OFF_VALUE);
> + wait_panel_status(intel_dp, IDLE_OFF_MASK, IDLE_OFF_VALUE, 10);
> }
>
> static void wait_panel_power_cycle(struct intel_dp *intel_dp)
> @@ -683,7 +702,7 @@ static void wait_panel_power_cycle(struct intel_dp *intel_dp)
> if (remaining)
> wait_remaining_ms_from_jiffies(jiffies, remaining);
>
> - wait_panel_status(intel_dp, IDLE_CYCLE_MASK, IDLE_CYCLE_VALUE);
> + wait_panel_status(intel_dp, IDLE_CYCLE_MASK, IDLE_CYCLE_VALUE, 1);
> }
>
> void intel_pps_wait_power_cycle(struct intel_dp *intel_dp)
--
Jani Nikula, Intel
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