Calling drm_atomic_helper_check_modeset() after drm_atomic_helper_check_planes()

Calling drm_atomic_helper_check_modeset() after drm_atomic_helper_check_planes()

[Jyri Sarha]

Hi,
The documentation of drm_atomic_helper_check_modeset() says:

"Drivers which update ->mode_changed (e.g. in their ->atomic_check hooks
if a plane update can't be done without a full modeset) _must_ call this
function afterwards after that change. It is permitted to call this
function multiple times for the same update, e.g. when the
->atomic_check functions depend upon the adjusted dotclock for fifo
space allocation and watermark computation."

Then drm_crtc_helper_funcs mode_fixup() callback documentations says:

"Atomic drivers which need to inspect and adjust more state should
instead use the @atomic_check callback."

Now an atomic driver that needs to do the both in the check phase:
update ->mode_changed and inspect and adjust the mode, is in trouble.
This is because any adjusted_mode modifications are reset by the
afterwards call of drm_atomic_helper_check_modeset().

I can get the atomic tilcdc driver working either: by using the
crtc_helper mode_fixup() calback, tuning the mode once more in
mode_set_nofb(), or calling drm_atomic_helper_check_modeset() last in
drm_mode_config_funcs atomic_check() callback.

What is the right thing to do? Any of these three approaches (and
possibly updating the documentation), or some other option that I did
not think of?

Best regards,
Jyri
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Re: Calling drm_atomic_helper_check_modeset() after drm_atomic_helper_check_planes()

[Daniel Vetter]

On Sun, Jun 19, 2016 at 9:57 PM, Jyri Sarha <jsarha@ti.com> wrote:
> Hi,
> The documentation of drm_atomic_helper_check_modeset() says:
>
> "Drivers which update ->mode_changed (e.g. in their ->atomic_check hooks
> if a plane update can't be done without a full modeset) _must_ call this
> function afterwards after that change. It is permitted to call this
> function multiple times for the same update, e.g. when the
> ->atomic_check functions depend upon the adjusted dotclock for fifo
> space allocation and watermark computation."
>
> Then drm_crtc_helper_funcs mode_fixup() callback documentations says:
>
> "Atomic drivers which need to inspect and adjust more state should
> instead use the @atomic_check callback."
>
> Now an atomic driver that needs to do the both in the check phase:
> update ->mode_changed and inspect and adjust the mode, is in trouble.
> This is because any adjusted_mode modifications are reset by the
> afterwards call of drm_atomic_helper_check_modeset().
>
> I can get the atomic tilcdc driver working either: by using the
> crtc_helper mode_fixup() calback, tuning the mode once more in
> mode_set_nofb(), or calling drm_atomic_helper_check_modeset() last in
> drm_mode_config_funcs atomic_check() callback.
>
> What is the right thing to do? Any of these three approaches (and
> possibly updating the documentation), or some other option that I did
> not think of?

Tuning the mode once more in mode_set_nofb is no-go - that's in the
commit phase, you're not allowed to change state objects then any
more. Also note that the hint for mode_fixup is just a "should", and
meant that way. You can split things up between mode_fixup and
atomic_check callbacks. Pick whatever works best for you from the
other 2 options.

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