linux-kernel - Re: [PATCH 1/6] drm: bridge: samsung-dsim: Support multi-lane calculations

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Message-ID: <6bfeb8aa-f4f4-4b24-a070-108e5553ad48@denx.de>
Date:   Mon, 17 Apr 2023 00:02:41 +0200
From:   Marek Vasut <marex@...x.de>
To:     Adam Ford <aford173@...il.com>, dri-devel@...ts.freedesktop.org
Cc:     m.szyprowski@...sung.com, aford@...conembedded.com,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Inki Dae <inki.dae@...sung.com>,
        Jagan Teki <jagan@...rulasolutions.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] drm: bridge: samsung-dsim: Support multi-lane
 calculations

On 4/15/23 12:40, Adam Ford wrote:
> If there is more than one lane, the HFP, HBP, and HSA is calculated in
> bytes/pixel, then they are divided amongst the different lanes with some
> additional overhead. This is necessary to achieve higher resolutions while
> keeping the pixel clocks lower as the number of lanes increase.
> 
> Signed-off-by: Adam Ford <aford173@...il.com>
> ---
>   drivers/gpu/drm/bridge/samsung-dsim.c | 40 +++++++++++++++++++++++----
>   1 file changed, 34 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
> index e0a402a85787..1ccbad4ea577 100644
> --- a/drivers/gpu/drm/bridge/samsung-dsim.c
> +++ b/drivers/gpu/drm/bridge/samsung-dsim.c
> @@ -215,6 +215,7 @@
>   #define DSI_RX_FIFO_SIZE		256
>   #define DSI_XFER_TIMEOUT_MS		100
>   #define DSI_RX_FIFO_EMPTY		0x30800002
> +#define DSI_HSYNC_PKT_OVERHEAD	6
>   
>   #define OLD_SCLK_MIPI_CLK_NAME		"pll_clk"
>   
> @@ -879,13 +880,40 @@ static void samsung_dsim_set_display_mode(struct samsung_dsim *dsi)
>   			| DSIM_MAIN_VBP(m->vtotal - m->vsync_end);
>   		samsung_dsim_write(dsi, DSIM_MVPORCH_REG, reg);
>   
> -		reg = DSIM_MAIN_HFP(m->hsync_start - m->hdisplay)
> -			| DSIM_MAIN_HBP(m->htotal - m->hsync_end);
> -		samsung_dsim_write(dsi, DSIM_MHPORCH_REG, reg);
> +		/*
> +		 * If there is more than one lane, the HFP, HBP, and HSA
> +		 * is calculated in bytes/pixel, then they are divided
> +		 * amongst the different lanes with some additional
> +		 * overhead correction
> +		 */

Did you find any confirmation of this in the MX8M* datasheet or at least 
by measuring the DSI data lanes with a scope ?

It would be real cool if this could be confirmed somehow, and we could 
rule out that this tweaking of HSA/HSE/... stuff isn't related to either 
LP-HS transition timing calculation this driver is missing, OR, 
incorrect flags in various bridge/panel drivers like commit:

ca161b259cc84 ("drm/bridge: ti-sn65dsi83: Do not generate HFP/HBP/HSA 
and EOT packet")