-----Original Message----- From: Nikula, Jani <[email protected]> Sent: Friday, September 12, 2025 1:56 AM To: Cavitt, Jonathan <[email protected]>; [email protected] Cc: Gupta, saurabhg <[email protected]>; Zuo, Alex <[email protected]>; Cavitt, Jonathan <[email protected]>; [email protected]; Manna, Animesh <[email protected]> Subject: Re: [PATCH] drm/i915/display: Simplify modular operations with vtotal > > On Thu, 11 Sep 2025, Jonathan Cavitt <[email protected]> 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 <[email protected]> > > Cc: Ville Syrjälä <[email protected]> > > Cc: Animesh Manna <[email protected]> > > Cc: Jani Nikula <[email protected]> > > --- > > 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. ... 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. -Jonathan Cavitt > > } > > > > /* > > -- > Jani Nikula, Intel >
