On Thu, 10 Feb 2022 at 07:59, Laurent Pinchart
<laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
>
> Hi Abhinav,
>
> On Wed, Feb 09, 2022 at 05:40:29PM -0800, Abhinav Kumar wrote:
> > Hi Laurent
> >
> > Gentle reminder on this.
>
> I won't have time before next week I'm afraid.
Laurent, another gentle ping.
>
> > On 2/6/2022 11:20 PM, Abhinav Kumar wrote:
> > > Hi Laurent
> > >
> > > On 2/6/2022 3:32 PM, Dmitry Baryshkov wrote:
> > >> On Wed, 2 Feb 2022 at 16:26, Laurent Pinchart
> > >> <laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
> > >>>
> > >>> Hi Jani,
> > >>>
> > >>> On Wed, Feb 02, 2022 at 03:15:03PM +0200, Jani Nikula wrote:
> > >>>> On Wed, 02 Feb 2022, Laurent Pinchart wrote:
> > >>>>> On Wed, Feb 02, 2022 at 02:24:28PM +0530, Kandpal Suraj wrote:
> > >>>>>> Changing rcar_du driver to accomadate the change of
> > >>>>>> drm_writeback_connector.base and drm_writeback_connector.encoder
> > >>>>>> to a pointer the reason for which is explained in the
> > >>>>>> Patch(drm: add writeback pointers to drm_connector).
> > >>>>>>
> > >>>>>> Signed-off-by: Kandpal Suraj <suraj.kandpal@xxxxxxxxx>
> > >>>>>> ---
> > >>>>>> drivers/gpu/drm/rcar-du/rcar_du_crtc.h | 2 ++
> > >>>>>> drivers/gpu/drm/rcar-du/rcar_du_writeback.c | 8 +++++---
> > >>>>>> 2 files changed, 7 insertions(+), 3 deletions(-)
> > >>>>>>
> > >>>>>> diff --git a/drivers/gpu/drm/rcar-du/rcar_du_crtc.h
> > >>>>>> b/drivers/gpu/drm/rcar-du/rcar_du_crtc.h
> > >>>>>> index 66e8839db708..68f387a04502 100644
> > >>>>>> --- a/drivers/gpu/drm/rcar-du/rcar_du_crtc.h
> > >>>>>> +++ b/drivers/gpu/drm/rcar-du/rcar_du_crtc.h
> > >>>>>> @@ -72,6 +72,8 @@ struct rcar_du_crtc {
> > >>>>>> const char *const *sources;
> > >>>>>> unsigned int sources_count;
> > >>>>>>
> > >>>>>> + struct drm_connector connector;
> > >>>>>> + struct drm_encoder encoder;
> > >>>>>
> > >>>>> Those fields are, at best, poorly named. Furthermore, there's no
> > >>>>> need in
> > >>>>> this driver or in other drivers using drm_writeback_connector to
> > >>>>> create
> > >>>>> an encoder or connector manually. Let's not polute all drivers because
> > >>>>> i915 doesn't have its abstractions right.
> > >>>>
> > >>>> i915 uses the quite common model for struct inheritance:
> > >>>>
> > >>>> struct intel_connector {
> > >>>> struct drm_connector base;
> > >>>> /* ... */
> > >>>> }
> > >>>>
> > >>>> Same with at least amd, ast, fsl-dcu, hisilicon, mga200, msm, nouveau,
> > >>>> radeon, tilcdc, and vboxvideo.
> > >>>>
> > >>>> We could argue about the relative merits of that abstraction, but I
> > >>>> think the bottom line is that it's popular and the drivers using it are
> > >>>> not going to be persuaded to move away from it.
> > >>>
> > >>> Nobody said inheritance is bad.
> > >>>
> > >>>> It's no coincidence that the drivers who've implemented writeback so
> > >>>> far
> > >>>> (komeda, mali, rcar-du, vc4, and vkms) do not use the abstraction,
> > >>>> because the drm_writeback_connector midlayer does, forcing the issue.
> > >>>
> > >>> Are you sure it's not a coincidence ? :-)
> > >>>
> > >>> The encoder and especially connector created by drm_writeback_connector
> > >>> are there only because KMS requires a drm_encoder and a drm_connector to
> > >>> be exposed to userspace (and I could argue that using a connector for
> > >>> writeback is a hack, but that won't change). The connector is "virtual",
> > >>> I still fail to see why i915 or any other driver would need to wrap it
> > >>> into something else. The whole point of the drm_writeback_connector
> > >>> abstraction is that drivers do not have to manage the writeback
> > >>> drm_connector manually, they shouldn't touch it at all.
> > >>
> > >> Laurent, I wanted to shift a bit from the question of drm_connector to
> > >> the question of drm_encoder being embedded in the
> > >> drm_writeback_connector.
> > >> In case of the msm driver the drm_encoder is not a lightweight entity,
> > >> but a full-featured driver part. Significant part of it can be shared
> > >> with the writeback implementation, if we allow using a pointer to the
> > >> external drm_encoder with the drm_writeback_connector.
> > >> Does the following patch set stand a chance to receive your ack?
> > >> - Switch drm_writeback_connector to point to drm_encoder rather than
> > >> embedding it?
> > >> - Create drm_encoder for the drm_writeback_connector when one is not
> > >> specified, so the current drivers can be left unchanged.
> > >>
> > >
> > > I second Dmitry's request here. For the reasons he has mentioned along
> > > with the possibility of the writeback encoder being shared across
> > > display pipelines, strengthens our request of the drm encoder being a
> > > pointer inside the drm_writeback_connector instead of embedding it.
> > >
> > > Like I had shown in my RFC, in case the other drivers dont specify one,
> > > we can allocate one:
> > >
> > > https://patchwork.kernel.org/project/dri-devel/patch/1642732195-25349-1-git-send-email-quic_abhinavk@xxxxxxxxxxx/
> > >
> > >
> > > We think this should be a reasonable accomodation to the existing
> > > drm_writeback driver.
> > >
> > > Thanks
> > >
> > > Abhinav
> > >
> > >>>
> > >>>> So I think drm_writeback_connector should *not* use the inheritance
> > >>>> abstraction because it's a midlayer that should leave that option to
> > >>>> the
> > >>>> drivers. I think drm_writeback_connector needs to be changed to
> > >>>> accommodate that, and, unfortunately, it means current writeback users
> > >>>> need to be changed as well.
> > >>>>
> > >>>> I am not sure, however, if the series at hand is the right
> > >>>> approach. Perhaps writeback can be modified to allocate the stuff for
> > >>>> you if you prefer it that way, as long as the drm_connector is not
> > >>>> embedded in struct drm_writeback_connector.
> > >>>>
> > >>>>> Nack.
> > >>>>>
> > >>>>>> struct drm_writeback_connector writeback;
> > >>>>>> };
> > >>>>>>
> > >>>>>> diff --git a/drivers/gpu/drm/rcar-du/rcar_du_writeback.c
> > >>>>>> b/drivers/gpu/drm/rcar-du/rcar_du_writeback.c
> > >>>>>> index c79d1259e49b..5b1e83380c47 100644
> > >>>>>> --- a/drivers/gpu/drm/rcar-du/rcar_du_writeback.c
> > >>>>>> +++ b/drivers/gpu/drm/rcar-du/rcar_du_writeback.c
> > >>>>>> @@ -200,8 +200,10 @@ int rcar_du_writeback_init(struct
> > >>>>>> rcar_du_device *rcdu,
> > >>>>>> {
> > >>>>>> struct drm_writeback_connector *wb_conn = &rcrtc->writeback;
> > >>>>>>
> > >>>>>> - wb_conn->encoder.possible_crtcs = 1 <<
> > >>>>>> drm_crtc_index(&rcrtc->crtc);
> > >>>>>> - drm_connector_helper_add(&wb_conn->base,
> > >>>>>> + wb_conn->base = &rcrtc->connector;
> > >>>>>> + wb_conn->encoder = &rcrtc->encoder;
> > >>>>>> + wb_conn->encoder->possible_crtcs = 1 <<
> > >>>>>> drm_crtc_index(&rcrtc->crtc);
> > >>>>>> + drm_connector_helper_add(wb_conn->base,
> > >>>>>> &rcar_du_wb_conn_helper_funcs);
> > >>>>>>
> > >>>>>> return drm_writeback_connector_init(&rcdu->ddev, wb_conn,
> > >>>>>> @@ -220,7 +222,7 @@ void rcar_du_writeback_setup(struct
> > >>>>>> rcar_du_crtc *rcrtc,
> > >>>>>> struct drm_framebuffer *fb;
> > >>>>>> unsigned int i;
> > >>>>>>
> > >>>>>> - state = rcrtc->writeback.base.state;
> > >>>>>> + state = rcrtc->writeback.base->state;
> > >>>>>> if (!state || !state->writeback_job)
> > >>>>>> return;
> > >>>>>>
>
> --
> Regards,
>
> Laurent Pinchart
--
With best wishes
Dmitry
