linux-kernel - Re: [PATCH] drm/sun4i: uncouple DSI dotclock divider from TCON0_DCLK

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Message-ID: <20230329195639.iep4rv5rcigu3gj2@penduick>
Date:   Wed, 29 Mar 2023 21:56:39 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Frank Oltmanns <frank@...manns.dev>
Cc:     Roman Beranek <romanberanek@...oud.com>,
        Chen-Yu Tsai <wens@...e.org>, David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Samuel Holland <samuel@...lland.org>,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/sun4i: uncouple DSI dotclock divider from
 TCON0_DCLK_REG

Hi,

On Tue, Mar 28, 2023 at 09:28:19PM +0200, Frank Oltmanns wrote:
> --- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> +++ b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> @@ -819,6 +819,34 @@ static void sun6i_dsi_encoder_disable(struct drm_encoder *encoder)
>  	regulator_disable(dsi->regulator);
>  }
> 
> +static bool sun6i_dsi_encoder_mode_fixup(
> +				   struct drm_encoder *encoder,
> +				   const struct drm_display_mode *mode,
> +				   struct drm_display_mode *adjusted_mode)

So, mode_fixup is kind of deprecated in favour of atomic_check

> +{
> +	if (encoder->encoder_type == DRM_MODE_ENCODER_DSI) {
> +		/*
> +		 * For DSI the PLL rate has to respect the bits per pixel and
> +		 * number of lanes.
> +		 *
> +		 * According to the BSP code:
> +		 * PLL rate = DOTCLOCK * bpp / lanes
> +		 *
> +		 * Therefore, the clock has to be adjusted in order to set the
> +		 * correct PLL rate when actually setting the clock.
> +		 */
> +		struct sun6i_dsi *dsi = encoder_to_sun6i_dsi(encoder);
> +		struct mipi_dsi_device *device = dsi->device;
> +		u8 bpp = mipi_dsi_pixel_format_to_bpp(device->format);
> +		u8 lanes = device->lanes;
> +
> +		adjusted_mode->crtc_clock = mode->crtc_clock
> +				 * bpp / (lanes * SUN6I_DSI_TCON_DIV);

And that's visible to the userspace, so it's not where we should store
that value. I guess the best way to do something similar would be to
store it into crtc_state, and then reuse it there. But it starts to make
a lot of rather complicated code compared to your previous patch.

Maxime

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