linux-kernel - Re: [PATCH 8/8] drm/tidss: Enable Dual and Duplicate Modes for OLDI

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Date:   Wed, 27 Jul 2022 16:22:50 +0300
From:   Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To:     Aradhya Bhatia <a-bhatia1@...com>, Jyri Sarha <jyri.sarha@....fi>,
        Rob Herring <robh+dt@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>
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>,
        Devicetree List <devicetree@...r.kernel.org>,
        Linux Kernel List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 8/8] drm/tidss: Enable Dual and Duplicate Modes for OLDI

Hi,

On 19/07/2022 11:08, Aradhya Bhatia wrote:
> The AM625 DSS peripheral supports 2 OLDI TXes which can work to enable 2
> duplicated displays of smaller resolutions or enable a single Dual-Link
> display with a higher resolution (1920x1200).
> 
> Configure the necessary register to enable the different modes.
> 
> Signed-off-by: Aradhya Bhatia <a-bhatia1@...com>
> ---
>   drivers/gpu/drm/tidss/tidss_dispc.c | 44 +++++++++++++++++++++++++++--
>   1 file changed, 41 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/tidss/tidss_dispc.c b/drivers/gpu/drm/tidss/tidss_dispc.c
> index 0b9689453ee8..28cb61259471 100644
> --- a/drivers/gpu/drm/tidss/tidss_dispc.c
> +++ b/drivers/gpu/drm/tidss/tidss_dispc.c
> @@ -1021,8 +1021,8 @@ static void dispc_enable_oldi(struct dispc_device *dispc, u32 hw_videoport,
>   	int count = 0;
>   
>   	/*
> -	 * For the moment DUALMODESYNC, MASTERSLAVE, MODE, and SRC
> -	 * bits of DISPC_VP_DSS_OLDI_CFG are set statically to 0.
> +	 * For the moment MASTERSLAVE, and SRC bits of DISPC_VP_DSS_OLDI_CFG are
> +	 * set statically to 0.
>   	 */
>   
>   	if (fmt->data_width == 24)
> @@ -1039,7 +1039,45 @@ static void dispc_enable_oldi(struct dispc_device *dispc, u32 hw_videoport,
>   
>   	oldi_cfg |= BIT(0); /* ENABLE */
>   
> -	dispc_vp_write(dispc, hw_videoport, DISPC_VP_DSS_OLDI_CFG, oldi_cfg);
> +	/*
> +	 * As per all the current implementations of DSS, the OLDI TXes are present only on
> +	 * hw_videoport = 0 (OLDI TX 0). However, the config register for 2nd OLDI TX (OLDI TX 1)
> +	 * is present in the address space of hw_videoport = 1. Hence, using "hw_videoport + 1" to
> +	 * configure OLDI TX 1.
> +	 */
> +
> +	switch (dispc->oldi_mode) {
> +	case OLDI_MODE_OFF:
> +		oldi_cfg &= ~BIT(0); /* DISABLE */
> +		dispc_vp_write(dispc, hw_videoport, DISPC_VP_DSS_OLDI_CFG, oldi_cfg);
> +		dispc_vp_write(dispc, hw_videoport + 1, DISPC_VP_DSS_OLDI_CFG, oldi_cfg);
> +		break;
> +
> +	case OLDI_SINGLE_LINK_SINGLE_MODE_0:
> +		dispc_vp_write(dispc, hw_videoport, DISPC_VP_DSS_OLDI_CFG, oldi_cfg);
> +		break;
> +
> +	case OLDI_SINGLE_LINK_SINGLE_MODE_1:
> +		dispc_vp_write(dispc, hw_videoport + 1, DISPC_VP_DSS_OLDI_CFG, oldi_cfg);
> +		break;
> +
> +	case OLDI_SINGLE_LINK_DUPLICATE_MODE:
> +		oldi_cfg |= BIT(5); /* DUPLICATE MODE */
> +		dispc_vp_write(dispc, hw_videoport, DISPC_VP_DSS_OLDI_CFG, oldi_cfg);
> +		dispc_vp_write(dispc, hw_videoport + 1, DISPC_VP_DSS_OLDI_CFG, oldi_cfg);
> +		break;
> +
> +	case OLDI_DUAL_LINK:
> +		oldi_cfg |= BIT(11); /* DUALMODESYNC */
> +		dispc_vp_write(dispc, hw_videoport, DISPC_VP_DSS_OLDI_CFG, oldi_cfg);
> +		dispc_vp_write(dispc, hw_videoport + 1, DISPC_VP_DSS_OLDI_CFG, oldi_cfg);
> +		break;
> +
> +	default:
> +		dev_warn(dispc->dev, "%s: Incorrect oldi mode. Returning.\n",
> +			 __func__);
> +		return;
> +	}
>   
>   	while (!(oldi_reset_bit & dispc_read(dispc, DSS_SYSSTATUS)) &&
>   	       count < 10000)

This feels a bit hacky:

- The function is dispc_enable_oldi, but the above code also disables 
oldi. We have code in dispc_vp_unprepare() which disables OLDI at the 
moment.

- The function takes hw_videoport as a parameter, and is designed to 
work on that videoport. The above operates on two videoports. Isn't the 
function also called for hw_videoport +1, which would result in reg 
writes to hw_videoport + 2?

- No matching code in dispc_vp_unprepare

Obviously the duplicate mode (I presume that's "cloning") and the dual 
link complicate things here, and I have to say I haven't worked with 
such setups. But I think somehow this should be restructured so that 
common configuration (common to the OLDIs) is done somewhere else.

I would guess that there are other drivers that support cloning and dual 
mode. Did you have a look how they handle things?

  Tomi