From mboxrd@z Thu Jan 1 00:00:00 1970 From: john@metanate.com (John Keeping) Date: Thu, 14 Jan 2016 16:26:19 +0000 Subject: [PATCH 3/3] drm/rockchip: explain why we can't wait_for_vblanks In-Reply-To: <20160114145705.GA24005@ulmo> References: <20160114142047.GD19130@phenom.ffwll.local> <107bbc36a316ed0ddc7b5a8bcd9b6db6cbc71d4f.1452782114.git.john@metanate.com> <20160114145705.GA24005@ulmo> Message-ID: <20160114162619.233ab39c.john@metanate.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Thu, 14 Jan 2016 15:57:05 +0100, Thierry Reding wrote: > On Thu, Jan 14, 2016 at 02:39:42PM +0000, John Keeping wrote: > > Signed-off-by: John Keeping > > --- > > drivers/gpu/drm/rockchip/rockchip_drm_fb.c | 6 ++++++ > > 1 file changed, 6 insertions(+) > > > > diff --git a/drivers/gpu/drm/rockchip/rockchip_drm_fb.c b/drivers/gpu/drm/rockchip/rockchip_drm_fb.c > > index 679d23a..b267ce4 100644 > > --- a/drivers/gpu/drm/rockchip/rockchip_drm_fb.c > > +++ b/drivers/gpu/drm/rockchip/rockchip_drm_fb.c > > @@ -177,6 +177,12 @@ static void rockchip_crtc_wait_for_update(struct drm_crtc *crtc) > > crtc_funcs->wait_for_update(crtc); > > } > > > > +/* > > + * We can't use drm_atomic_helper_wait_for_vblanks() because rk3288 and rk3066 > > + * have hardware counters for neither vblanks nor scanlines. This function is > > + * equivalent but uses rockchip_crtc_wait_for_update() instead of waiting for > > + * vblank_count to change. > > + */ > > This is kind of misleading. From reading earlier parts of the thread the > reason why drm_atomic_helper_wait_for_vblanks() won't work is because it > has a potential race condition that can't be detected unless you also > have a vblank counter. However, the above comment makes it work like > drm_atomic_helper_wait_for_vblanks() doesn't work in the absence of a > vblank counter, which isn't quite true. How about something like this (using the sequence from Mark's message): /* * We can't use drm_atomic_helper_wait_for_vblanks() because rk3288 and rk3066 * have hardware counters for neither vblanks nor scanlines, which results in * a race where: * | <-- HW vsync irq and reg take effect * plane_commit --> | * get_vblank and wait --> | * | <-- handle_vblank, vblank->count + 1 * cleanup_fb --> | * iommu crash --> | * | <-- HW vsync irq and reg take effect * * This function is equivalent but uses rockchip_crtc_wait_for_update() instead * of waiting for vblank_count to change. */