-----Original Message----- From: Ville Syrjälä <ville.syrj...@linux.intel.com> Sent: Friday, September 12, 2025 8:30 AM To: Cavitt, Jonathan <jonathan.cav...@intel.com> Cc: Nikula, Jani <jani.nik...@intel.com>; intel-gfx@lists.freedesktop.org; Gupta, saurabhg <saurabhg.gu...@intel.com>; Zuo, Alex <alex....@intel.com>; Manna, Animesh <animesh.ma...@intel.com> Subject: Re: [PATCH] drm/i915/display: Simplify modular operations with vtotal > > On Fri, Sep 12, 2025 at 02:29:17PM +0000, Cavitt, Jonathan wrote: > > -----Original Message----- > > From: Nikula, Jani <jani.nik...@intel.com> > > Sent: Friday, September 12, 2025 1:56 AM > > To: Cavitt, Jonathan <jonathan.cav...@intel.com>; > > intel-gfx@lists.freedesktop.org > > Cc: Gupta, saurabhg <saurabhg.gu...@intel.com>; Zuo, Alex > > <alex....@intel.com>; Cavitt, Jonathan <jonathan.cav...@intel.com>; > > ville.syrj...@linux.intel.com; Manna, Animesh <animesh.ma...@intel.com> > > Subject: Re: [PATCH] drm/i915/display: Simplify modular operations with > > vtotal > > > > > > On Thu, 11 Sep 2025, Jonathan Cavitt <jonathan.cav...@intel.com> wrote: > > > > There are a couple of modulus operations in the i915 display code with > > > > vtotal as the divisor that add vtotal to the dividend. In modular > > > > arithmetic, adding the divisor to the dividend is equivalent to adding > > > > zero to the dividend, so this addition can be dropped. > > > > > > The result might become negative with this? > > > > > > BR, > > > Jani. > > > > > > > > > > > Signed-off-by: Jonathan Cavitt <jonathan.cav...@intel.com> > > > > Cc: Ville Syrjälä <ville.syrj...@linux.intel.com> > > > > Cc: Animesh Manna <animesh.ma...@intel.com> > > > > Cc: Jani Nikula <jani.nik...@intel.com> > > > > --- > > > > drivers/gpu/drm/i915/display/intel_dsb.c | 4 ++-- > > > > drivers/gpu/drm/i915/display/intel_vblank.c | 2 +- > > > > 2 files changed, 3 insertions(+), 3 deletions(-) > > > > > > > > diff --git a/drivers/gpu/drm/i915/display/intel_dsb.c > > > > b/drivers/gpu/drm/i915/display/intel_dsb.c > > > > index dee44d45b668..67315116839b 100644 > > > > --- a/drivers/gpu/drm/i915/display/intel_dsb.c > > > > +++ b/drivers/gpu/drm/i915/display/intel_dsb.c > > > > @@ -173,7 +173,7 @@ static int dsb_scanline_to_hw(struct > > > > intel_atomic_state *state, > > > > intel_pre_commit_crtc_state(state, crtc); > > > > int vtotal = dsb_vtotal(state, crtc); > > > > > > > > - return (scanline + vtotal - > > > > intel_crtc_scanline_offset(crtc_state)) % vtotal; > > > > + return (scanline - intel_crtc_scanline_offset(crtc_state)) % > > > > vtotal; > > > > intel_crtc_scanline_offset returns -1, 1, or 2. So the result here could > > only be negative if > > the value of scanline is less than 2. > > > > > > } > > > > > > > > /* > > > > @@ -482,7 +482,7 @@ static void assert_dsl_ok(struct intel_atomic_state > > > > *state, > > > > * Waiting for the entire frame doesn't make sense, > > > > * (IN==don't wait, OUT=wait forever). > > > > */ > > > > - drm_WARN(crtc->base.dev, (end - start + vtotal) % vtotal == > > > > vtotal - 1, > > > > + drm_WARN(crtc->base.dev, (end - start) % vtotal == vtotal - 1, > > > > This can only be negative if start is less than end, which doesn't seem > > possible. > > > > > > "[CRTC:%d:%s] DSB %d bad scanline window wait: %d-%d > > > > (vt=%d)\n", > > > > crtc->base.base.id, crtc->base.name, dsb->id, > > > > start, end, vtotal); > > > > diff --git a/drivers/gpu/drm/i915/display/intel_vblank.c > > > > b/drivers/gpu/drm/i915/display/intel_vblank.c > > > > index c15234c1d96e..bcfca2fcef3c 100644 > > > > --- a/drivers/gpu/drm/i915/display/intel_vblank.c > > > > +++ b/drivers/gpu/drm/i915/display/intel_vblank.c > > > > @@ -288,7 +288,7 @@ static int __intel_get_crtc_scanline(struct > > > > intel_crtc *crtc) > > > > * See update_scanline_offset() for the details on the > > > > * scanline_offset adjustment. > > > > */ > > > > - return (position + vtotal + crtc->scanline_offset) % vtotal; > > > > + return (position + crtc->scanline_offset) % vtotal; > > > > crtc->scanline_offset = intel_crtc_scanline_offset(crtc_state). > > And position = intel_de_read_fw(display, PIPEDSL(display, pipe)) & > > PIPEDSL_LINE_MASK. > > Finally, #define PIPEDSL_LINE_MASK REG_GENMASK(19, 0) > > So, unless position = 0 on display versions 1 or 2 (where > > intel_crtc_scanline_offset returns -1), this cannot be negative. > > Scanlines can be anything from 0 to vtotal-1. > So nak on this patch. > > > > > ... > > Wait, if crtc->scanline_offset = intel_crtc_scanline_offset(crtc_state), > > then why are we recalculating > > it in dsb_scanline_to_hw? That should also probably be fixed, but not in > > this patch. > > Not sure what you think needs fixing. dsb_scanline_to_hw() is the > inverse of most other uses of scanline_offset.
Well, yes, we subtract it instead of adding it. But like, in dsb_scanline_to_hw: """ return (scanline + vtotal - intel_crtc_scanline_offset(crtc_state)) % vtotal; """ Can this not be simplified to: """ return (scanline + vtotal - crtc->scanline_offset) % vtotal; """ ? -Jonathan Cavitt > > > -Jonathan Cavitt > > > > > > } > > > > > > > > /* > > > > > > -- > > > Jani Nikula, Intel > > > > > -- > Ville Syrjälä > Intel >
