The self-refresh helper framework overloads "disable" to sometimes mean
"go into self-refresh mode," and this mode activates automatically
(e.g., after some period of unchanging display output). In such cases,
the display pipe is still considered "on", and user-space is not aware
that we went into self-refresh mode. Thus, users may expect that
vblank-related features (such as DRM_IOCTL_WAIT_VBLANK) still work
properly.

However, we trigger the WARN_ONCE() here if a CRTC driver tries to leave
vblank enabled.

Add a different expectation: that CRTCs *should* leave vblank enabled
when going into self-refresh.

This patch is preparation for another patch -- "drm/rockchip: vop: Leave
vblank enabled in self-refresh" -- which resolves conflicts between the
above self-refresh behavior and the API tests in IGT's kms_vblank test
module.

== Some alternatives discussed: ==

It's likely that on many display controllers, vblank interrupts will
turn off when the CRTC is disabled, and so in some cases, self-refresh
may not support vblank. To support such cases, we might consider
additions to the generic helpers such that we fire vblank events based
on a timer.

However, there is currently only one driver using the common
self-refresh helpers (i.e., rockchip), and at least as of commit
bed030a4 ("drm/rockchip: Don't fully disable vop on self refresh"),
the CRTC hardware is powered enough to continue to generate vblank
interrupts.

So we chose the simpler option of leaving vblank interrupts enabled. We
can reevaluate this decision and perhaps augment the helpers if/when we
gain a second driver that has different requirements.

v3:
 * include discussion summary

v2:
 * add 'ret != 0' warning case for self-refresh
 * describe failing test case and relation to drm/rockchip patch better

Cc: <stable@vger.kernel.org> # dependency for "drm/rockchip: vop: Leave
                             # vblank enabled in self-refresh"
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230109171809.v3.1.I3904f697863649eb1be540ecca147a66e42bfad7@changeid