Re: [Freedreno] [PATCH v4 03/20] drm: allow real encoder to be passed for drm_writeback_connector

[Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx>]

On 4/25/2022 3:50 AM, Dmitry Baryshkov wrote:
> On Sun, 24 Apr 2022 at 22:59, Laurent Pinchart
> <laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
> > Hi Abhinav,
> >
> > On Sun, Apr 24, 2022 at 11:23:20AM -0700, Abhinav Kumar wrote:
> > > On 4/24/2022 11:12 AM, Abhinav Kumar wrote:
> > > > On 4/24/2022 7:50 AM, Laurent Pinchart wrote:
> > > > > On Fri, Apr 22, 2022 at 04:06:38PM -0700, Abhinav Kumar wrote:
> > > > > > For some vendor driver implementations, display hardware can
> > > > > > be shared between the encoder used for writeback and the physical
> > > > > > display.
> > > > > >
> > > > > > In addition resources such as clocks and interrupts can
> > > > > > also be shared between writeback and the real encoder.
> > > > > >
> > > > > > To accommodate such vendor drivers and hardware, allow
> > > > > > real encoder to be passed for drm_writeback_connector.
> > > > > >
> > > > > > For existing clients, drm_writeback_connector_init() will use
> > > > > > an internal_encoder under the hood and hence no changes will
> > > > > > be needed.
> > > > > >
> > > > > > changes in v7:
> > > > > >      - move this change before the vc4 change in the series
> > > > > >        to minimize the changes to vendor drivers in drm core
> > > > > >        changes
> > > > >
> > > > > Why is this needed ? The drm_writeback_connector functions don't need
> > > > > the drm_encoder after drm_writeback_connector_init() (or
> > > > > drm_writeback_connector_init_with_encoder()) returns.
> > > >
> > > > Sorry I didnt follow this comment. This change log is incorrect, so
> > > > after changing the previous change in the series and modifying this, no
> > > > further changes are needed to vc4, so I decided to drop the next change.
> > > > So this change log is incorrect. I can remove this.
> > > >
> > > > Is that what you meant?
> > >
> > > So till the previous change, the only user of
> > > drm_writeback_connector_init_with_encoder() was
> > > drm_writeback_connector_init() which was still passing its own
> > > internal_encoder.
> > >
> > > Only if the wb_connector->encoder is changed to a pointer, other vendor
> > > drivers can pass their own encoder to
> > > drm_writeback_connector_init_with_encoder().
> > >
> > > Hence you are right that drm_writeback_connector functions do not need
> > > drm_encoder after init() returns, but till this change is done, other
> > > vendor drivers cannot directly call
> > > drm_writeback_connector_init_with_encoder() because the encoder will not
> > > be valid till then.
> >
> > Users of drm_writeback_connector_init_with_encoder() handle the encoder
> > themselves, they can simply ignore drm_writeback_connector.encoder. The
> > documentation of the encoder field needs to be updated though (I'd do so
> > in the previous patch), to clearly mention that the field is valid only
> > when using drm_writeback_connector_init(), not when calling
> > drm_writeback_connector_init_with_encoder().
>
> If we allow it to be unitialized, it might end with hard-to-trace
> bugs, additional conditions, etc.
> In my opnion we should:
>   - either make drm_writeback_connector.encoder a valid pointer
>   - or drop the field completely.
>
> And up to some point I'd vote for the second option. The code using
> internal_encoder can continue using it directly. The code using
> drm_writeback_connector_init_with_encoder() will manage encoder on
> their own. We will loose a single entry point for wb's encoder, but do
> we really need it? (Frankly speaking I didn't check.)
Agree with your comment Dmitry and thats why I was suggesting to let 
drm_writeback_connector.encoder remain a valid pointer to avoid 
discrepancies.

If we go with option (1), this change as-it-is is the best way to go.

If we go with option (2), to drop the encoder field and just rename 
encoder ---> internal_encoder, it will still work for existing users.

But I would prefer that goes as a separate change then because MSM 
doesn't really need this ( and at this point even other drivers don't )

I am still ready to post the change separately and do basic validation 
with our boards.

> > > Hope this clarifies it.
> > >
> > > > > > Signed-off-by: Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx>
> > > > > > Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx>
> > > > > > ---
> > > > > >    drivers/gpu/drm/drm_writeback.c | 18 ++++++++++++------
> > > > > >    drivers/gpu/drm/vc4/vc4_txp.c   |  4 ++--
> > > > > >    include/drm/drm_writeback.h     | 22 ++++++++++++++++++++--
> > > > > >    3 files changed, 34 insertions(+), 10 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/gpu/drm/drm_writeback.c
> > > > > > b/drivers/gpu/drm/drm_writeback.c
> > > > > > index 92658ad..0538674 100644
> > > > > > --- a/drivers/gpu/drm/drm_writeback.c
> > > > > > +++ b/drivers/gpu/drm/drm_writeback.c
> > > > > > @@ -180,21 +180,21 @@ int drm_writeback_connector_init(struct
> > > > > > drm_device *dev,
> > > > > >    {
> > > > > >        int ret = 0;
> > > > > > -    drm_encoder_helper_add(&wb_connector->encoder, enc_helper_funcs);
> > > > > > +    drm_encoder_helper_add(&wb_connector->internal_encoder, enc_helper_funcs);
> > > > > > -    wb_connector->encoder.possible_crtcs = possible_crtcs;
> > > > > > +    wb_connector->internal_encoder.possible_crtcs = possible_crtcs;
> > > > > > -    ret = drm_encoder_init(dev, &wb_connector->encoder,
> > > > > > +    ret = drm_encoder_init(dev, &wb_connector->internal_encoder,
> > > > > >                       &drm_writeback_encoder_funcs,
> > > > > >                       DRM_MODE_ENCODER_VIRTUAL, NULL);
> > > > > >        if (ret)
> > > > > >            return ret;
> > > > > > -    ret = drm_writeback_connector_init_with_encoder(dev, wb_connector, &wb_connector->encoder,
> > > > > > -            con_funcs, formats, n_formats);
> > > > > > +    ret = drm_writeback_connector_init_with_encoder(dev, wb_connector,
> > > > > > +            &wb_connector->internal_encoder, con_funcs, formats, n_formats);
> > > > > >        if (ret)
> > > > > > -        drm_encoder_cleanup(&wb_connector->encoder);
> > > > > > +        drm_encoder_cleanup(&wb_connector->internal_encoder);
> > > > > >        return ret;
> > > > > >    }
> > > > > > @@ -239,6 +239,12 @@ int drm_writeback_connector_init_with_encoder(struct drm_device *dev,
> > > > > >        struct drm_mode_config *config = &dev->mode_config;
> > > > > >        int ret = create_writeback_properties(dev);
> > > > > > +    /*
> > > > > > +     * Assign the encoder passed to this API to the wb_connector's encoder.
> > > > > > +     * For drm_writeback_connector_init(), this shall be the internal_encoder
> > > > > > +     */
> > > > > > +    wb_connector->encoder = enc;
> > > > > > +
> > > > > >        if (ret != 0)
> > > > > >            return ret;
> > > > > > diff --git a/drivers/gpu/drm/vc4/vc4_txp.c b/drivers/gpu/drm/vc4/vc4_txp.c
> > > > > > index 3447eb6..7e063a9 100644
> > > > > > --- a/drivers/gpu/drm/vc4/vc4_txp.c
> > > > > > +++ b/drivers/gpu/drm/vc4/vc4_txp.c
> > > > > > @@ -159,7 +159,7 @@ struct vc4_txp {
> > > > > >    static inline struct vc4_txp *encoder_to_vc4_txp(struct drm_encoder *encoder)
> > > > > >    {
> > > > > > -    return container_of(encoder, struct vc4_txp, connector.encoder);
> > > > > > +    return container_of(encoder, struct vc4_txp, connector.internal_encoder);
> > > > > >    }
> > > > > >    static inline struct vc4_txp *connector_to_vc4_txp(struct  drm_connector *conn)
> > > > > > @@ -507,7 +507,7 @@ static int vc4_txp_bind(struct device *dev, struct device *master, void *data)
> > > > > >        if (ret)
> > > > > >            return ret;
> > > > > > -    encoder = &txp->connector.encoder;
> > > > > > +    encoder = txp->connector.encoder;
> > > > > >        encoder->possible_crtcs = drm_crtc_mask(crtc);
> > > > > >        ret = devm_request_irq(dev, irq, vc4_txp_interrupt, 0,
> > > > > > diff --git a/include/drm/drm_writeback.h b/include/drm/drm_writeback.h
> > > > > > index bb306fa..3fbae9d 100644
> > > > > > --- a/include/drm/drm_writeback.h
> > > > > > +++ b/include/drm/drm_writeback.h
> > > > > > @@ -25,13 +25,31 @@ struct drm_writeback_connector {
> > > > > >        struct drm_connector base;
> > > > > >        /**
> > > > > > -     * @encoder: Internal encoder used by the connector to fulfill
> > > > > > +     * @encoder: handle to drm_encoder used by the connector to fulfill
> > > > > >         * the DRM framework requirements. The users of the
> > > > > >         * @drm_writeback_connector control the behaviour of the @encoder
> > > > > >         * by passing the @enc_funcs parameter to drm_writeback_connector_init()
> > > > > >         * function.
> > > > > > +     *
> > > > > > +     * For some vendor drivers, the hardware resources are shared between
> > > > > > +     * writeback encoder and rest of the display pipeline.
> > > > > > +     * To accommodate such cases, encoder is a handle to the real encoder
> > > > > > +     * hardware.
> > > > > > +     *
> > > > > > +     * For current existing writeback users, this shall continue to be the
> > > > > > +     * embedded encoder for the writeback connector.
> > > > > > +     */
> > > > > > +    struct drm_encoder *encoder;
> > > > > > +
> > > > > > +    /**
> > > > > > +     * @internal_encoder: internal encoder used by writeback when
> > > > > > +     * drm_writeback_connector_init() is used.
> > > > > > +     * @encoder will be assigned to this for those cases
> > > > > > +     *
> > > > > > +     * This will be unused when drm_writeback_connector_init_with_encoder()
> > > > > > +     * is used.
> > > > > >         */
> > > > > > -    struct drm_encoder encoder;
> > > > > > +    struct drm_encoder internal_encoder;
> > > > > >        /**
> > > > > >         * @pixel_formats_blob_ptr:
> >
> > --
> > Regards,
> >
> > Laurent Pinchart