[PATCH v2] drm/tidss: dispc: Rewrite naive plane positioning code

[Jyri Sarha]

On 12/02/2020 22:28, Daniel Vetter wrote:
> On Wed, Feb 12, 2020 at 7:01 PM Jyri Sarha <jsarha at ti.com> wrote:
>>
>> On 12/02/2020 16:33, Ville Syrjälä wrote:
>>> On Wed, Feb 12, 2020 at 04:08:11PM +0200, Jyri Sarha wrote:
>>>> On 12/02/2020 15:59, Jyri Sarha wrote:
>>>>> The old implementation of placing planes on the CRTC while configuring
>>>>> the planes was naive and relied on the order in which the planes were
>>>>> configured, enabled, and disabled. The situation where a plane's zpos
>>>>> was changed on the fly was completely broken. The usual symptoms of
>>>>> this problem was scrambled display and a flood of sync lost errors,
>>>>> when a plane was active in two layers at the same time, or a missing
>>>>> plane, in case when a layer was accidentally disabled.
>>>>>
>>>>> The rewrite takes a more straight forward approach when HW is
>>>>> concerned. The plane positioning registers are in the CRTC (actually
>>>>> OVR) register space and it is more natural to configure them in one go
>>>>> while configuring the CRTC. To do this we need to make sure we have
>>>>> all the planes on updated CRTCs in the new atomic state to be
>>>>> committed. This is done by calling drm_atomic_add_affected_planes() in
>>>>> crtc_atomic_check().
>>>>>
>>>>> Signed-off-by: Jyri Sarha <jsarha at ti.com>
>>>>> ---
>>>>>  drivers/gpu/drm/tidss/tidss_crtc.c  | 55 ++++++++++++++++++++++++++++-
>>>>>  drivers/gpu/drm/tidss/tidss_dispc.c | 55 +++++++++++------------------
>>>>>  drivers/gpu/drm/tidss/tidss_dispc.h |  5 +++
>>>>>  3 files changed, 79 insertions(+), 36 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/tidss/tidss_crtc.c b/drivers/gpu/drm/tidss/tidss_crtc.c
>>>>> index 032c31ee2820..f7c5fd1094a8 100644
>>>>> --- a/drivers/gpu/drm/tidss/tidss_crtc.c
>>>>> +++ b/drivers/gpu/drm/tidss/tidss_crtc.c
>>>> ...
>>>>> @@ -108,7 +110,54 @@ static int tidss_crtc_atomic_check(struct drm_crtc *crtc,
>>>>>             return -EINVAL;
>>>>>     }
>>>>>
>>>>> -   return dispc_vp_bus_check(dispc, hw_videoport, state);
>>>>> +   ret = dispc_vp_bus_check(dispc, hw_videoport, state);
>>>>> +   if (ret)
>>>>> +           return ret;
>>>>> +
>>>>> +   /* Add unchanged planes on this crtc to state for zpos update. */
>>>>> +   return drm_atomic_add_affected_planes(state->state, crtc);
>>>>
>>>> Is this a correct way to use drm_atomic_add_affected_planes()?
>>>>
>>>> I saw that some other drivers implement their own mode_config
>>>> atomic_check() and have this call there in
>>>> for_each_new_crtc_in_state()-loop, but I thought it should be fine to
>>>> call it in crtc_atomic_check().
>>>
>>> You seem to be using drm_atomic_helper_check_planes(), which means
>>> crtc.atomic_check() gets called after plane.atomic_check(). So this
>>> might be good or bad depending on whether you'd like the planes you
>>> add here to go through their .atomic_check() or not.
>>>
>>
>> Should have thought of that my self. Extra plane.atomic_check() calls do
>> not do any actual harm, but they are potentially expensive. The planes
>> are really only needed there in the commit phase (crtc_enable() or
>> flush()). Well, I'll do my own mode_config.atomic_check() and call
>> drm_atomic_add_affected_planes() in a for_each_new_crtc_in_state()-loop
>> after all the checks.
> 
> Also, if you do use the helpers then this should already have happened
> for you. Which makes me wonder why all this work, so maybe there's
> some dependency between all the various check functions you have going
> on? Might be time to roll your own top-level check function that calls

> stuff in the order your hw needs things to be checked, so that you
> don't add new states late and have to run one check phase twice (which
> is totally fine with the helpers as long as all your check callbacks
> are idempotent, but often just overkill and confusing).

All the driver specific checks are perfectly independent without any
cross dependencies. And there is no extra data in the plane- or
CRTC-state, nor do the driver specific checks touch the state in any way.

I only want all the planes on a crtc to be there, if any of the planes
on the CRTC changes, so that I can write the plane positions from the
scratch directly from the atomic state with each commit.

Long explanation (if you are interested):

With the DSS HW the planes are positioned to CRTCs by filling each
plane's id and x,y position to correct overlay register according to the
plane's zpos (each overlay reg has its own static zpos).

Using the naive implementation, there is a problem if I have plane0 at
zpos0 and another commit comes with plane1 at zpos0 and plane0 disabled.
If plane1 in the commit is handled first, it overwrites plane0's
previous position, and plane0 handled afterwards will disable freshly
configured plane1. There is number of other problematic cases.

Ok, I can easily fix this by storing the plane positions (x,y, and z) in
the driver's internal state, and rolling the positions out in the
crtc_enable() or -flush(). But I have understood the atomic drivers
should avoid having any internal state. So the obvious choice would be
to roll out the plane positions directly from the atomic state. However,
there is a problem with that.

The problem appears when I have more than one plane active on a crtc and
I just need to update one of the planes. In the situation nothing
changes about the CRTC and the unchanged planes, so it is quite
understandable that the helpers do not add the unchanged planes to the
atomic state. But if I want to roll out the new plane positions from the
scratch with every commit that touches any plane on that CRTC, I need to
have the unchanged planes there.

So the drm_atomic_add_affected_planes()-calls are added just to avoid
any internal plane position state in the driver.

Best regards,
Jyri

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