Re: [PATCH] drm/bridge: tc358767: Support input format negotiation hook

[Sam Ravnborg <sam@ravnborg.org>]

Hi Aradhya,

On Tue, Nov 07, 2023 at 01:17:03AM +0530, Aradhya Bhatia wrote:
> Hi Sam,
> 
> Thank you for the suggestion!
> 
> On 06-Nov-23 18:08, Sam Ravnborg wrote:
> > Hi Aradhya,
> > 
> > On Tue, Oct 31, 2023 at 12:58:46AM +0530, Aradhya Bhatia wrote:
> >> With new connector model, tc358767 will not create the connector, when
> >> DRM_BRIDGE_ATTACH_NO_CONNECTOR is set and display-controller driver will
> >> rely on format negotiation to setup the encoder format.
> >>
> >> Add the missing input-format negotiation hook in the
> >> drm_bridge_funcs to complete DRM_BRIDGE_ATTACH_NO_CONNECTOR support.
> >>
> >> Input format is selected to MEDIA_BUS_FMT_RGB888_1X24 as default, as is
> >> the case with older model.
> >>
> >> Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
> >> Signed-off-by: Aradhya Bhatia <a-bhatia1@ti.com>
> >> ---
> >>
> >> Notes:
> >>
> >>   * Since I do not have hardware with me, this was just build tested. I would
> >>     appreciate it if someone could test and review it, especically somebody, who
> >>     uses the bridge for DPI/DSI to eDP format conversion.
> >>
> >>   * The Toshiba TC358767 bridge is not enabled in arm64 defconfig by default,
> >>     when it should be. Hence, I sent a quick patch[0] earlier.
> >>
> >> [0]: https://lore.kernel.org/all/20231030152834.18450-1-a-bhatia1@ti.com/
> >>
> >>  drivers/gpu/drm/bridge/tc358767.c | 25 +++++++++++++++++++++++++
> >>  1 file changed, 25 insertions(+)
> >>
> >> diff --git a/drivers/gpu/drm/bridge/tc358767.c b/drivers/gpu/drm/bridge/tc358767.c
> >> index ef2e373606ba..0affcefdeb1c 100644
> >> --- a/drivers/gpu/drm/bridge/tc358767.c
> >> +++ b/drivers/gpu/drm/bridge/tc358767.c
> >> @@ -1751,6 +1751,30 @@ tc_dpi_atomic_get_input_bus_fmts(struct drm_bridge *bridge,
> >>  	return input_fmts;
> >>  }
> >>  
> >> +static u32 *
> >> +tc_edp_atomic_get_input_bus_fmts(struct drm_bridge *bridge,
> >> +				 struct drm_bridge_state *bridge_state,
> >> +				 struct drm_crtc_state *crtc_state,
> >> +				 struct drm_connector_state *conn_state,
> >> +				 u32 output_fmt,
> >> +				 unsigned int *num_input_fmts)
> >> +{
> >> +	u32 *input_fmts;
> >> +
> >> +	*num_input_fmts = 0;
> >> +
> >> +	input_fmts = kcalloc(MAX_INPUT_SEL_FORMATS, sizeof(*input_fmts),
> >> +			     GFP_KERNEL);
> >> +	if (!input_fmts)
> >> +		return NULL;
> >> +
> >> +	/* This is the DSI/DPI-end bus format */
> >> +	input_fmts[0] = MEDIA_BUS_FMT_RGB888_1X24;
> >> +	*num_input_fmts = 1;
> >> +
> >> +	return input_fmts;
> >> +}
> > 
> > You could benefit from using the helper:
> > drm_atomic_helper_bridge_propagate_bus_fmt()
> 
> You are right!
> 
> Upon taking a second look, I realize that the bridge chain works with
> MEDIA_BUS_FMT_FIXED bus format, when tc358767 is being used in DPI/DSI
> to eDP mode (because the panel-bridge does not have a get_output_bus_fmt
> hook, and uses the same helper for its get_input_bus_fmt hook). My patch
> creates a deviation from that, by forcing MEDIA_BUS_FMT_RGB888_1X24 even
> when eDP is involved.
> 
> Using the helper here, will certainly address this deviation.
> 
> However, for the DPI/DSI to DP mode, MEDIA_BUS_FMT_RGB888_1X24 bus
> format is required, and *just* using the helper as its get_input_bus_fmt
> hook, might not be enough.
> 
> Since tc358767 is the last bridge in DPI/DSI to DP mode, the
> output_fmt parameter get defaulted to MEDIA_BUS_FMT_FIXED too, as there
> is no get_output_bus_fmt hook present in the driver. If we simply us
> the helper here, the input_fmt will also get set to MEDIA_BUS_FMT_FIXED.
> This too is an unwanted deviation.
> 
> It seems like the right way to address both the cases, would be by
> adding the get_output_bus_fmt hook that sets output_fmt to
> MEDIA_BUS_FMT_RGB888_1X24, as well as using the helper as the
> get_input_bus_fmt hook.
> 
> If this seems good to you too, I will send a new version of Tomi's
> series[0] which incorporates this patch.

I never managed to fully wrap my head around the bus fmt negotiation,
and as I am trying to recover from a flu this is not the time to try.
Your explanations sounds like you have grasped it so I suggest to move
ahead.

	Sam