linux-kernel - Re: [PATCH 4/8] drm/tidss: Add support for Dual Link LVDS Bus Format

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Date:   Thu, 28 Jul 2022 14:45:34 +0300
From:   Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To:     Aradhya Bhatia <a-bhatia1@...com>
Cc:     Darren Etheridge <detheridge@...com>, Nishanth Menon <nm@...com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Rahul T R <r-ravikumar@...com>,
        Krunal Bhargav <k-bhargav@...com>,
        Devarsh Thakkar <devarsht@...com>,
        DRI Development List <dri-devel@...ts.freedesktop.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Rob Herring <robh+dt@...nel.org>,
        Jyri Sarha <jyri.sarha@....fi>,
        Devicetree List <devicetree@...r.kernel.org>,
        Linux Kernel List <linux-kernel@...r.kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>
Subject: Re: [PATCH 4/8] drm/tidss: Add support for Dual Link LVDS Bus Format

On 28/07/2022 14:03, Tomi Valkeinen wrote:
> On 19/07/2022 11:08, Aradhya Bhatia wrote:
>> The 2 OLDI TXes in the AM625 SoC can be synced together to output a 2K
>> resolution video.
>>
>> Add support in the driver for the discovery of such a dual mode
>> connection on the OLDI video port, using the values of "ti,oldi-mode"
>> property.
>>
>> Signed-off-by: Aradhya Bhatia <a-bhatia1@...com>
>> ---
>>   drivers/gpu/drm/tidss/tidss_dispc.c | 39 +++++++++++++++++++++--------
>>   1 file changed, 28 insertions(+), 11 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/tidss/tidss_dispc.c 
>> b/drivers/gpu/drm/tidss/tidss_dispc.c
>> index add725fa682b..fb1fdecfc83a 100644
>> --- a/drivers/gpu/drm/tidss/tidss_dispc.c
>> +++ b/drivers/gpu/drm/tidss/tidss_dispc.c
>> @@ -853,25 +853,36 @@ void dispc_set_irqenable(struct dispc_device 
>> *dispc, dispc_irq_t mask)
>>       }
>>   }
>> -enum dispc_oldi_mode_reg_val { SPWG_18 = 0, JEIDA_24 = 1, SPWG_24 = 2 };
>> +enum dispc_oldi_mode_reg_val {
>> +    SPWG_18        = 0,
>> +    JEIDA_24    = 1,
>> +    SPWG_24        = 2,
>> +    DL_SPWG_18    = 4,
>> +    DL_JEIDA_24    = 5,
>> +    DL_SPWG_24    = 6,
>> +};
>>   struct dispc_bus_format {
>>       u32 bus_fmt;
>>       u32 data_width;
>>       bool is_oldi_fmt;
>> +    bool is_dual_link;
>>       enum dispc_oldi_mode_reg_val oldi_mode_reg_val;
>>   };
>>   static const struct dispc_bus_format dispc_bus_formats[] = {
>> -    { MEDIA_BUS_FMT_RGB444_1X12,        12, false, 0 },
>> -    { MEDIA_BUS_FMT_RGB565_1X16,        16, false, 0 },
>> -    { MEDIA_BUS_FMT_RGB666_1X18,        18, false, 0 },
>> -    { MEDIA_BUS_FMT_RGB888_1X24,        24, false, 0 },
>> -    { MEDIA_BUS_FMT_RGB101010_1X30,        30, false, 0 },
>> -    { MEDIA_BUS_FMT_RGB121212_1X36,        36, false, 0 },
>> -    { MEDIA_BUS_FMT_RGB666_1X7X3_SPWG,    18, true, SPWG_18 },
>> -    { MEDIA_BUS_FMT_RGB888_1X7X4_SPWG,    24, true, SPWG_24 },
>> -    { MEDIA_BUS_FMT_RGB888_1X7X4_JEIDA,    24, true, JEIDA_24 },
>> +    { MEDIA_BUS_FMT_RGB444_1X12,        12, false, false, 0 },
>> +    { MEDIA_BUS_FMT_RGB565_1X16,        16, false, false, 0 },
>> +    { MEDIA_BUS_FMT_RGB666_1X18,        18, false, false, 0 },
>> +    { MEDIA_BUS_FMT_RGB888_1X24,        24, false, false, 0 },
>> +    { MEDIA_BUS_FMT_RGB101010_1X30,        30, false, false, 0 },
>> +    { MEDIA_BUS_FMT_RGB121212_1X36,        36, false, false, 0 },
>> +    { MEDIA_BUS_FMT_RGB666_1X7X3_SPWG,    18, true, false, SPWG_18 },
>> +    { MEDIA_BUS_FMT_RGB888_1X7X4_SPWG,    24, true, false, SPWG_24 },
>> +    { MEDIA_BUS_FMT_RGB888_1X7X4_JEIDA,    24, true, false, JEIDA_24 },
>> +    { MEDIA_BUS_FMT_RGB666_1X7X3_SPWG,    18, true, true, DL_SPWG_18 },
>> +    { MEDIA_BUS_FMT_RGB888_1X7X4_SPWG,    24, true, true, DL_SPWG_24 },
>> +    { MEDIA_BUS_FMT_RGB888_1X7X4_JEIDA,    24, true, true, 
>> DL_JEIDA_24 },
>>   };
> 
> So the dual link sends two pixels per clock, right? Are there panel or 
> bridge drivers that support this? My initial thought was that it should 
> be a new bus format.

Looks like we have drm bridges supporting dual link, and they use the 
"normal" bus format. Did you have a look at them? They require two port 
nodes for dual link, and use the existence of the second one to decide 
if dual link is used or not.

There are also lvds helpers in drm_of.c. I didn't look closely, but it 
looked to me that the helpers can tell you if the ports are connected to 
a dual link bridge. If not, you could fall back to cloning. This way no 
extra properties are needed. But you will need to add a port node, which 
I think you need to add anyway for cloning.

  Tomi