Re: [RFC PATCH 0/3] Support for Solid Fill Planes

[Dmitry Baryshkov <dmitry.baryshkov@linaro.org>]

On 28/06/2023 00:27, Jessica Zhang wrote:
> 
> 
> On 6/27/2023 12:58 AM, Pekka Paalanen wrote:
>> On Mon, 26 Jun 2023 16:02:50 -0700
>> Jessica Zhang <quic_jesszhan@quicinc.com> wrote:
>>
>>> On 11/7/2022 11:37 AM, Ville Syrjälä wrote:
>>>> On Fri, Oct 28, 2022 at 03:59:49PM -0700, Jessica Zhang wrote:
>>>>> Introduce and add support for COLOR_FILL and COLOR_FILL_FORMAT
>>>>> properties. When the color fill value is set, and the framebuffer 
>>>>> is set
>>>>> to NULL, memory fetch will be disabled.
>>>>
>>>> Thinking a bit more universally I wonder if there should be
>>>> some kind of enum property:
>>>>
>>>> enum plane_pixel_source {
>>>>     FB,
>>>>     COLOR,
>>>>     LIVE_FOO,
>>>>     LIVE_BAR,
>>>>     ...
>>>> }
>>>
>>> Reviving this thread as this was the initial comment suggesting to
>>> implement pixel_source as an enum.
>>>
>>> I think the issue with having pixel_source as an enum is how to decide
>>> what counts as a NULL commit.
>>>
>>> Currently, setting the FB to NULL will disable the plane. So I'm
>>> guessing we will extend that logic to "if there's no pixel_source set
>>> for the plane, then it will be a NULL commit and disable the plane".
>>>
>>> In that case, the question then becomes when to set the pixel_source to
>>> NONE. Because if we do that when setting a NULL FB (or NULL solid_fill
>>> blob), it then forces userspace to set one property before the other.
>>
>> Right, that won't work.
>>
>> There is no ordering between each property being set inside a single
>> atomic commit. They can all be applied to kernel-internal state
>> theoretically simultaneously, or any arbitrary random order, and the
>> end result must always be the same. Hence, setting one property cannot
>> change the state of another mutable property. I believe that doing
>> otherwise would make userspace fragile and hard to get right.
>>
>> I guess there might be an exception to that rule when the same property
>> is set multiple times in a single atomic commit; the last setting in
>> the array prevails. That's universal and not a special-case between two
>> specific properties.
>>
>>> Because of that, I'm thinking of having pixel_source be represented by a
>>> bitmask instead. That way, we will simply unset the corresponding
>>> pixel_source bit when passing in a NULL FB/solid_fill blob. Then, in
>>> order to detect whether a commit is NULL or has a valid pixel source, we
>>> can just check if pixel_source == 0.
>>
>> Sounds fine to me at first hand, but isn't there the enum property that
>> says if the kernel must look at solid_fill blob *or* FB_ID?
>>
>> If enum prop says "use solid_fill prop", the why would changes to FB_ID
>> do anything? Is it for backwards-compatibility with KMS clients that do
>> not know about the enum prop?
>>
>> It seems like that kind of backwards-compatiblity will cause problems
>> in trying to reason about the atomic state, as explained above, leading
>> to very delicate and fragile conditions where things work intuitively.
>> Hence, I'm not sure backwards-compatibility is wanted. This won't be
>> the first or the last KMS property where an unexpected value left over
>> will make old atomic KMS clients silently malfunction up to showing no
>> recognisable picture at all. *If* that problem needs solving, there
>> have been ideas floating around about resetting everything to nice
>> values so that userspace can ignore what it does not understand. So far
>> there has been no real interest in solving that problem in the kernel
>> though.
>>
>> Legacy non-atomic UAPI wrappers can do whatever they want, and program
>> any (new) properties they want in order to implement the legacy
>> expectations, so that does not seem to be a problem.
> 
> Hi Pekka and Dmitry,
> 
> After reading through both of your comments, I think I have a better 
> understanding of the pixel_source implementation now.
> 
> So to summarize, we want to expose another property called 
> "pixel_source" to userspace that will default to FB (as to not break 
> legacy userspace).
> 
> If userspace wants to use solid fill planes, it will set both the 
> solid_fill *and* pixel_source properties to a valid blob and COLOR 
> respectively. If it wants to use FB, it will set FB_ID and pixel_source 
> to a valid FB and FB.
> 
> Here's a table illustrating what I've described above:
> 
> +-----------------+-------------------------+-------------------------+
> | Use Case        | Legacy Userspace        | solid_fill-aware        |
> |                 |                         | Userspace               |
> +=================+=========================+=========================+
> | Valid FB        | pixel_source = FB       | pixel_source = FB       |
> |                 | FB_ID = valid FB        | FB_ID = valid FB        |
> +-----------------+-------------------------+-------------------------+
> | Valid           | pixel_source = COLOR    | N/A                     |
> | solid_fill blob | solid_fill = valid blob |                         |

Probably these two cells were swapped.

> +-----------------+-------------------------+-------------------------+
> | NULL commit     | pixel_source = FB       | pixel_source = FB       |
> |                 | FB_ID = NULL            | FB_ID = NULL            |
> +-----------------+-------------------------+-------------------------+

                                               | or:
                                               | pixel_source = COLOR
                                               | solid_fill = NULL
> 
> Is there anything I'm missing or needs to be clarified?
> 

LGTM otherwise

> Thanks,
> 
> Jessica Zhang
> 
>>
>>
>> Thanks,
>> pq
>>
>>
>>>
>>> Would be interested in any feedback on this.
>>>
>>> Thanks,
>>>
>>> Jessica Zhang
>>>
>>>>> In addition, loosen the NULL FB checks within the atomic commit 
>>>>> callstack
>>>>> to allow a NULL FB when color_fill is nonzero and add FB checks in
>>>>> methods where the FB was previously assumed to be non-NULL.
>>>>>
>>>>> Finally, have the DPU driver use drm_plane_state.color_fill and
>>>>> drm_plane_state.color_fill_format instead of 
>>>>> dpu_plane_state.color_fill,
>>>>> and add extra checks in the DPU atomic commit callstack to account 
>>>>> for a
>>>>> NULL FB in cases where color_fill is set.
>>>>>
>>>>> Some drivers support hardware that have optimizations for solid fill
>>>>> planes. This series aims to expose these capabilities to userspace as
>>>>> some compositors have a solid fill flag (ex. SOLID_COLOR in the 
>>>>> Android
>>>>> hardware composer HAL) that can be set by apps like the Android Gears
>>>>> app.
>>>>>
>>>>> Userspace can set the color_fill value by setting COLOR_FILL_FORMAT 
>>>>> to a
>>>>> DRM format, setting COLOR_FILL to a color fill value, and setting the
>>>>> framebuffer to NULL.
>>>>
>>>> Is there some real reason for the format property? Ie. why not
>>>> just do what was the plan for the crttc background color and
>>>> specify the color in full 16bpc format and just pick as many
>>>> msbs from that as the hw can use?
>>>>>
>>>>> Jessica Zhang (3):
>>>>>     drm: Introduce color fill properties for drm plane
>>>>>     drm: Adjust atomic checks for solid fill color
>>>>>     drm/msm/dpu: Use color_fill property for DPU planes
>>>>>
>>>>>    drivers/gpu/drm/drm_atomic.c              | 68 
>>>>> ++++++++++++-----------
>>>>>    drivers/gpu/drm/drm_atomic_helper.c       | 34 +++++++-----
>>>>>    drivers/gpu/drm/drm_atomic_uapi.c         |  8 +++
>>>>>    drivers/gpu/drm/drm_blend.c               | 38 +++++++++++++
>>>>>    drivers/gpu/drm/drm_plane.c               |  8 +--
>>>>>    drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c  |  7 ++-
>>>>>    drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c | 66 
>>>>> ++++++++++++++--------
>>>>>    include/drm/drm_atomic_helper.h           |  5 +-
>>>>>    include/drm/drm_blend.h                   |  2 +
>>>>>    include/drm/drm_plane.h                   | 28 ++++++++++
>>>>>    10 files changed, 188 insertions(+), 76 deletions(-)
>>>>>
>>>>> -- 
>>>>> 2.38.0
>>>>
>>>> -- 
>>>> Ville Syrjälä
>>>> Intel
>>

-- 
With best wishes
Dmitry