[PATCH 1/6] drm: allow real encoder to be passed for drm_writeback_connector

[Abhinav Kumar]

Hi Dmitry and Laurent

On 3/11/2022 12:05 AM, Laurent Pinchart wrote:
> On Fri, Mar 11, 2022 at 10:46:13AM +0300, Dmitry Baryshkov wrote:
>> On Fri, 11 Mar 2022 at 04:50, Abhinav Kumar <quic_abhinavk at quicinc.com> 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.
>>>
>>> Co-developed-by: Kandpal Suraj <suraj.kandpal at intel.com>
>>> Signed-off-by: Abhinav Kumar <quic_abhinavk at quicinc.com>
>>> ---
>>>   drivers/gpu/drm/drm_writeback.c |  8 ++++----
>>>   include/drm/drm_writeback.h     | 13 +++++++++++--
>>>   2 files changed, 15 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_writeback.c b/drivers/gpu/drm/drm_writeback.c
>>> index dccf4504..4dad687 100644
>>> --- a/drivers/gpu/drm/drm_writeback.c
>>> +++ b/drivers/gpu/drm/drm_writeback.c
>>> @@ -189,8 +189,8 @@ int drm_writeback_connector_init(struct drm_device *dev,
>>>          if (IS_ERR(blob))
>>>                  return PTR_ERR(blob);
>>>
>>> -       drm_encoder_helper_add(&wb_connector->encoder, enc_helper_funcs);
>>> -       ret = drm_encoder_init(dev, &wb_connector->encoder,
>>> +       drm_encoder_helper_add(wb_connector->encoder, enc_helper_funcs);
>>> +       ret = drm_encoder_init(dev, wb_connector->encoder,
>>>                                 &drm_writeback_encoder_funcs,
>>>                                 DRM_MODE_ENCODER_VIRTUAL, NULL);
>>
>> If the encoder is provided by a separate driver, it might use a
>> different set of encoder funcs.
> 
> More than that, if the encoder is provided externally but doesn't have
> custom operations, I don't really see the point of having an external
> encoder in the first place.
> 
> Has this series been tested with a driver that needs to provide an
> encoder, to make sure it fits the purpose ?
> 

Yes, I have tested this with the MSM driver which provides an encoder
and yes it absolutely fits the purpose.


>> I'd suggest checking whether the wb_connector->encoder is NULL here.
>> If it is, allocate one using drmm_kzalloc and init it.
>> If it is not NULL, assume that it has been initialized already, so
>> skip the drm_encoder_init() and just call the drm_encoder_helper_add()

You are both right. We can skip the drm_encoder_init for drivers which 
have already provided an encoder.

The only issue I was facing with that is some of the drivers for example 
the below one, access the "wb_conn->encoder.possible_crtcs" before the 
call to drm_writeback_connector_init().

198 int rcar_du_writeback_init(struct rcar_du_device *rcdu,
199 			   struct rcar_du_crtc *rcrtc)
200 {
201 	struct drm_writeback_connector *wb_conn = &rcrtc->writeback;
202
203 	wb_conn->encoder.possible_crtcs = 1 << drm_crtc_index(&rcrtc->crtc);
204 	drm_connector_helper_add(&wb_conn->base,
205 				 &rcar_du_wb_conn_helper_funcs);
206
207 	return drm_writeback_connector_init(&rcdu->ddev, wb_conn,
208 					    &rcar_du_wb_conn_funcs,
209 					    &rcar_du_wb_enc_helper_funcs,
210 					    writeback_formats,
211 					    ARRAY_SIZE(writeback_formats));

If we allocate the encoder within drm_writeback_connector_init(), do you 
suggest I modify the drivers to move the usage of possible_crtcs after 
the drm_writeback_connector_init() call to avoid NULL ptr crash?


>>
>>>          if (ret)
>>> @@ -204,7 +204,7 @@ int drm_writeback_connector_init(struct drm_device *dev,
>>>                  goto connector_fail;
>>>
>>>          ret = drm_connector_attach_encoder(connector,
>>> -                                               &wb_connector->encoder);
>>> +                                               wb_connector->encoder);
>>>          if (ret)
>>>                  goto attach_fail;
>>>
>>> @@ -233,7 +233,7 @@ int drm_writeback_connector_init(struct drm_device *dev,
>>>   attach_fail:
>>>          drm_connector_cleanup(connector);
>>>   connector_fail:
>>> -       drm_encoder_cleanup(&wb_connector->encoder);
>>> +       drm_encoder_cleanup(wb_connector->encoder);
>>>   fail:
>>>          drm_property_blob_put(blob);
>>>          return ret;
>>> diff --git a/include/drm/drm_writeback.h b/include/drm/drm_writeback.h
>>> index 9697d27..0ba266e 100644
>>> --- a/include/drm/drm_writeback.h
>>> +++ b/include/drm/drm_writeback.h
>>> @@ -25,13 +25,22 @@ 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;
>>> +       struct drm_encoder *encoder;
>>>
>>>          /**
>>>           * @pixel_formats_blob_ptr:
>