[PATCH 1/2] dt-bindings: drm: bridge: adi,adv7533: Document adi,disable-lanes-override property

[Biju Das]

Hi Laurent,

Thanks for the feedback.

> Subject: Re: [PATCH 1/2] dt-bindings: drm: bridge: adi,adv7533: Document
> adi,disable-lanes-override property
> 
> Hi Biju,
> 
> Thank you for the patch.
> 
> On Wed, Mar 09, 2022 at 03:11:08PM +0000, Biju Das wrote:
> > On Renesas RZ/{G2L,V2L} platforms changing the lanes from 4 to 3 at
> > lower frequencies causes display instability. On such platforms, it is
> > better to avoid switching lanes from 4 to 3 as it needs different set
> > of PLL parameter constraints to make the display stable with 3 lanes.
> 
> Is this because the PLL calculation code doesn't work properly, or because
> the hardware can't support this ?

PLL Calculation is correct, that is the reason it works for all resolution
with 4 lanes. There are 2 clocks generated by PLL5 which is connected to a mux
with clock sources, namely 'FOUTPOSTDIV' and 'FOUT1PH0'

This Mux is connected to DSI dividers. 

'FOUTPOSTDIV' should be selected if (PLL_INPUT_FREQ/VCLK) is odd and 'FOUT1PH0' should
be selected if it is even.

The PLL calculation makes use of even selection('FOUT1PH0') and video works for all frequencies
with 4 lanes.

With 'FOUT1PH0' as clock source, if I switch to 3 lanes for lanes it doesn't work. 
But it work with 4lanes on all frequencies.

HW can support 3 lanes, if I set parameter to make (PLL_INPUT_FREQ/VCLK) odd and select
'FOUTPOSTDIV' as clk source to DSI divider.

I am not sure about the rational behind the constraint to Switch to 3 lanes for
lower frequency for this ADV7535 chip, as on our platform it can work with 4lanes
on all frequencies.

Cheers,
Biju

> 
> > This patch introduces 'adi,disable-lanes-override' property to disable
> > lane switching at lower frequencies.
> >
> > Signed-off-by: Biju Das <biju.das.jz at bp.renesas.com>
> > Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj at bp.renesas.com>
> > ---
> >  .../devicetree/bindings/display/bridge/adi,adv7533.yaml      | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git
> > a/Documentation/devicetree/bindings/display/bridge/adi,adv7533.yaml
> > b/Documentation/devicetree/bindings/display/bridge/adi,adv7533.yaml
> > index f36209137c8a..2dc378039d21 100644
> > ---
> > a/Documentation/devicetree/bindings/display/bridge/adi,adv7533.yaml
> > +++ b/Documentation/devicetree/bindings/display/bridge/adi,adv7533.yam
> > +++ l
> > @@ -84,6 +84,11 @@ properties:
> >        timings for HDMI output.
> >      type: boolean
> >
> > +  adi,disable-lanes-override:
> > +    description:
> > +      Disables the overriding lanes at lower frequencies.
> > +    type: boolean
> > +
> >    adi,dsi-lanes:
> >      description: Number of DSI data lanes connected to the DSI host.
> >      $ref: /schemas/types.yaml#/definitions/uint32
> 
> --
> Regards,
> 
> Laurent Pinchart