Re: [PATCH 1/5] drm: Add atomic helper to redo a modeset on current mode

From: Manasi Navare <manasi.d.navare@intel.com>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@intel.com>,
	intel-gfx <intel-gfx@lists.freedesktop.org>,
	dri-devel <dri-devel@lists.freedesktop.org>
Subject: Re: [PATCH 1/5] drm: Add atomic helper to redo a modeset on current mode
Date: Mon, 24 Oct 2016 15:08:48 -0700	[thread overview]
Message-ID: <20161024220847.GA4039@intel.com> (raw)
In-Reply-To: <CAKMK7uEQL2eX6FYojJcF1fFv=Y6MLGR6z6XnVrBUhHgbZtERDQ@mail.gmail.com>

On Mon, Oct 24, 2016 at 09:12:35AM +0200, Daniel Vetter wrote:
> On Mon, Oct 24, 2016 at 9:00 AM, Manasi Navare
> <manasi.d.navare@intel.com> wrote:
> >> I guess we just need to do some additional work on top to make sure the
> >> vblank ioctl can't see invalid state. Which would then again make upfront
> >> link trainig possible ...

Thanks for the explanation. So the atomic_commit code in the driver
calls drm_crtc_send_vblank_event(crtc, crtc->state->event);, is this
what needs to be filtered to be seen by Vblank IOCTL?

> >
> > Could you share some more details on what work would need to be done
> > for vblank? Then I can investigate it a little bit more tomor during the day.
> 
> So the trouble is that drm_irq.c is still from the old pre-kms days.
> Which means it deals in int pipe instead of struct drm_crtc *, among
> other things. But we can't simply switch over because some old drivers
> (pre-kms ones) still depend upon that old code. Hence we need:
> 
> 1. Duplicate most of the code in drm_irq.c into a new
> drm_crtc_vblank.c, which is only used for kms drivers. And in the
> ioctl code have a DRIVER_MODESET check and either call the new kms
> version (in drm_crtc_vblank.c) or the old one (still in drm_irq.c).

What functions need to be duplicated?

Which IOCTL are we talking about here?  Do you mean in the atomic_commit_tail
where we check for driver modeset, is where we should then call the new kms version
of drm_crtc_send_vblank_event()?
> 
> 2. Convert the int pipe into a drm_crtc pointer in the new kms code.
> For prettiness we could (try) to do that as much as possible.
> 
> 3. Grab the drm_crtc->mutex in the ioctl code to block these races.

Again which IOCTL needs to grab the drm_crtc->mutex?

What is the end goal of thsi task, what race conditions are we trying to avoid
in case of these modesets during link training failures?

Manasi

> 
> -Daniel
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch

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