Re: [Intel-xe] [PATCH v4] drm/i915: handle uncore spinlock when not available

[Rodrigo Vivi <rodrigo.vivi@intel.com>]

On Wed, Nov 29, 2023 at 11:17:28AM +0200, Luca Coelho wrote:
> The uncore code may not always be available (e.g. when we build the
> display code with Xe), so we can't always rely on having the uncore's
> spinlock.
> 
> To handle this, split the spin_lock/unlock_irqsave/restore() into
> spin_lock/unlock() followed by a call to local_irq_save/restore() and
> create wrapper functions for locking and unlocking the uncore's
> spinlock.  In these functions, we have a condition check and only
> actually try to lock/unlock the spinlock when I915 is defined, and
> thus uncore is available.
> 
> This keeps the ifdefs contained in these new functions and all such
> logic inside the display code.
> 
> Cc: Tvrtko Ursulin <tvrto.ursulin@intel.com>
> Cc: Jani Nikula <jani.nikula@intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> Cc: Ville Syrj?l? <ville.syrjala@linux.intel.com>
> Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
> ---
> 
> In v2:
> 
>    * Renamed uncore_spin_*() to intel_spin_*()
>    * Corrected the order: save, lock, unlock, restore
> 
> In v3:
> 
>    * Undid the change to pass drm_i915_private instead of the lock
>      itself, since we would have to include i915_drv.h and that pulls
>      in a truckload of other includes.
> 
> In v4:
> 
>    * After a brief attempt to replace this with a different patch,
>      we're back to this one;
>    * Pass drm_i195_private again, and move the functions to
>      intel_vblank.c, so we don't need to include i915_drv.h in a
>      header file and it's already included in intel_vblank.c;
> 
>  drivers/gpu/drm/i915/display/intel_display.h |  1 +
>  drivers/gpu/drm/i915/display/intel_vblank.c  | 45 +++++++++++++++-----
>  2 files changed, 36 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_display.h b/drivers/gpu/drm/i915/display/intel_display.h
> index 8548f49e3972..5ff299bc4b87 100644
> --- a/drivers/gpu/drm/i915/display/intel_display.h
> +++ b/drivers/gpu/drm/i915/display/intel_display.h
> @@ -29,6 +29,7 @@
>  
>  #include "i915_reg_defs.h"
>  #include "intel_display_limits.h"
> +#include "i915_drv.h"

please move this include to intel_vblank.c

>  
>  enum drm_scaling_filter;
>  struct dpll;
> diff --git a/drivers/gpu/drm/i915/display/intel_vblank.c b/drivers/gpu/drm/i915/display/intel_vblank.c
> index 2cec2abf9746..d9625db82681 100644
> --- a/drivers/gpu/drm/i915/display/intel_vblank.c
> +++ b/drivers/gpu/drm/i915/display/intel_vblank.c
> @@ -265,6 +265,26 @@ int intel_crtc_scanline_to_hw(struct intel_crtc *crtc, int scanline)
>  	return (scanline + vtotal - crtc->scanline_offset) % vtotal;
>  }
>  
> +/*
> + * The uncore version of the spin lock functions is used to decide
> + * whether we need to lock the uncore lock or not.  This is only
> + * needed in i915, not in Xe.  Keep the decision-making centralized
> + * here.

maybe we could add brief mention that it is only needed because old hardware
that is not supported by Xe.

> + */
> +static inline void intel_vblank_section_enter(struct drm_i915_private *i915)

let's avoid inline here.

> +{
> +#ifdef I915
> +	spin_lock(&i915->uncore.lock);
> +#endif
> +}
> +
> +static inline void intel_vblank_section_exit(struct drm_i915_private *i915)

and here

With these changes:

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

> +{
> +#ifdef I915
> +	spin_unlock(&i915->uncore.lock);
> +#endif
> +}
> +
>  static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
>  				     bool in_vblank_irq,
>  				     int *vpos, int *hpos,
> @@ -302,11 +322,12 @@ static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
>  	}
>  
>  	/*
> -	 * Lock uncore.lock, as we will do multiple timing critical raw
> -	 * register reads, potentially with preemption disabled, so the
> -	 * following code must not block on uncore.lock.
> +	 * Enter vblank critical section, as we will do multiple
> +	 * timing critical raw register reads, potentially with
> +	 * preemption disabled, so the following code must not block.
>  	 */
> -	spin_lock_irqsave(&dev_priv->uncore.lock, irqflags);
> +	local_irq_save(irqflags);
> +	intel_vblank_section_enter(dev_priv);
>  
>  	/* preempt_disable_rt() should go right here in PREEMPT_RT patchset. */
>  
> @@ -374,7 +395,8 @@ static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
>  
>  	/* preempt_enable_rt() should go right here in PREEMPT_RT patchset. */
>  
> -	spin_unlock_irqrestore(&dev_priv->uncore.lock, irqflags);
> +	intel_vblank_section_exit(dev_priv);
> +	local_irq_restore(irqflags);
>  
>  	/*
>  	 * While in vblank, position will be negative
> @@ -412,9 +434,13 @@ int intel_get_crtc_scanline(struct intel_crtc *crtc)
>  	unsigned long irqflags;
>  	int position;
>  
> -	spin_lock_irqsave(&dev_priv->uncore.lock, irqflags);
> +	local_irq_save(irqflags);
> +	intel_vblank_section_enter(dev_priv);
> +
>  	position = __intel_get_crtc_scanline(crtc);
> -	spin_unlock_irqrestore(&dev_priv->uncore.lock, irqflags);
> +
> +	intel_vblank_section_exit(dev_priv);
> +	local_irq_restore(irqflags);
>  
>  	return position;
>  }
> @@ -537,7 +563,7 @@ void intel_crtc_update_active_timings(const struct intel_crtc_state *crtc_state,
>  	 * Need to audit everything to make sure it's safe.
>  	 */
>  	spin_lock_irqsave(&i915->drm.vblank_time_lock, irqflags);
> -	spin_lock(&i915->uncore.lock);
> +	intel_vblank_section_enter(i915);
>  
>  	drm_calc_timestamping_constants(&crtc->base, &adjusted_mode);
>  
> @@ -546,7 +572,6 @@ void intel_crtc_update_active_timings(const struct intel_crtc_state *crtc_state,
>  	crtc->mode_flags = mode_flags;
>  
>  	crtc->scanline_offset = intel_crtc_scanline_offset(crtc_state);
> -
> -	spin_unlock(&i915->uncore.lock);
> +	intel_vblank_section_exit(i915);
>  	spin_unlock_irqrestore(&i915->drm.vblank_time_lock, irqflags);
>  }
> -- 
> 2.39.2
>