linux-kernel - Re: [PATCH 1/2] drm/bridge: lt8912b: add support for P/N pin swap

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Date: Tue, 2 Apr 2024 18:53:07 +0200
From: Francesco Dolcini <francesco@...cini.it>
To: Alexandru Ardelean <alex@...uggie.ro>
Cc: linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	devicetree@...r.kernel.org, adrien.grassein@...il.com,
	andrzej.hajda@...el.com, neil.armstrong@...aro.org,
	rfoss@...nel.org, Laurent.pinchart@...asonboard.com,
	jonas@...boo.se, jernej.skrabec@...il.com, airlied@...il.com,
	daniel@...ll.ch, maarten.lankhorst@...ux.intel.com,
	mripard@...nel.org, tzimmermann@...e.de, robh@...nel.org,
	krzysztof.kozlowski+dt@...aro.org, conor+dt@...nel.org,
	stefan.eichenberger@...adex.com, francesco.dolcini@...adex.com,
	marius.muresan@....ro, irina.muresan@....ro
Subject: Re: [PATCH 1/2] drm/bridge: lt8912b: add support for P/N pin swap

Hello Alexandru, thanks for your patch.

On Tue, Apr 02, 2024 at 01:59:24PM +0300, Alexandru Ardelean wrote:
> On some HW designs, it's easier for the layout if the P/N pins are swapped.
> In those cases, we need to adjust (for this) by configuring the MIPI analog
> registers differently. Specifically, register 0x3e needs to be 0xf6
> (instead of 0xd6).
> 
> This change adds a 'lontium,pn-swap' device-tree property to configure the
> MIPI analog registers for P/N swap.
> 
> Signed-off-by: Alexandru Ardelean <alex@...uggie.ro>
> ---
>  drivers/gpu/drm/bridge/lontium-lt8912b.c | 25 +++++++++++++++++++++---
>  1 file changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/lontium-lt8912b.c b/drivers/gpu/drm/bridge/lontium-lt8912b.c
> index 4b2ae27f0a57f..154126bb922b4 100644
> --- a/drivers/gpu/drm/bridge/lontium-lt8912b.c
> +++ b/drivers/gpu/drm/bridge/lontium-lt8912b.c
> @@ -47,6 +47,7 @@ struct lt8912 {
>  
>  	u8 data_lanes;
>  	bool is_power_on;
> +	bool do_pn_swap;
>  };
>  
>  static int lt8912_write_init_config(struct lt8912 *lt)
> @@ -78,15 +79,31 @@ static int lt8912_write_init_config(struct lt8912 *lt)
>  		{0x55, 0x44},
>  		{0x57, 0x01},
>  		{0x5a, 0x02},
> -
> -		/*MIPI Analog*/
> +	};
> +	const struct reg_sequence mipi_analog_seq[] = {
>  		{0x3e, 0xd6},
>  		{0x3f, 0xd4},
>  		{0x41, 0x3c},
>  		{0xB2, 0x00},
>  	};
> +	const struct reg_sequence mipi_analog_pn_swap_seq[] = {
> +		{0x3e, 0xf6},
> +		{0x3f, 0xd4},
> +		{0x41, 0x3c},
> +		{0xB2, 0x00},
> +	};
> +	int ret;
>  
> -	return regmap_multi_reg_write(lt->regmap[I2C_MAIN], seq, ARRAY_SIZE(seq));
> +	ret = regmap_multi_reg_write(lt->regmap[I2C_MAIN], seq, ARRAY_SIZE(seq));
> +	if (ret < 0)
> +		return ret;
> +
> +	if (!lt->do_pn_swap)
> +		return regmap_multi_reg_write(lt->regmap[I2C_MAIN], mipi_analog_seq,
> +					      ARRAY_SIZE(mipi_analog_seq));
> +
> +	return regmap_multi_reg_write(lt->regmap[I2C_MAIN], mipi_analog_pn_swap_seq,
> +				      ARRAY_SIZE(mipi_analog_pn_swap_seq));

Can you just remove {0x3e, 0xd6} from the register/value array and write
it afterward depending on `do_pn_swap` value? Or keep it with the
current value and only overwrite it when do_pn_swap is true?

If you do it this way is a 4 line change.


>  static int lt8912_write_mipi_basic_config(struct lt8912 *lt)
> @@ -702,6 +719,8 @@ static int lt8912_parse_dt(struct lt8912 *lt)
>  	}
>  	lt->gp_reset = gp_reset;
>  
> +	lt->do_pn_swap = device_property_read_bool(dev, "lontium,pn-swap");

I would call this variable the same that is called in the lontium
documentation, mipirx_diff_swap

Francesco