linux-kernel - Re: [PATCH v9 2/3] drm: bridge: Add support for Cadence MHDP8546 DPI/DP bridge

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Message-ID: <e53e87b0-7e0a-763f-8b8b-0dc278e1f225@ti.com>
Date:   Tue, 1 Sep 2020 11:34:45 +0300
From:   Tomi Valkeinen <tomi.valkeinen@...com>
To:     Swapnil Jakhade <sjakhade@...ence.com>, <airlied@...ux.ie>,
        <daniel@...ll.ch>, <Laurent.pinchart@...asonboard.com>,
        <robh+dt@...nel.org>, <a.hajda@...sung.com>,
        <narmstrong@...libre.com>, <jonas@...boo.se>,
        <jernej.skrabec@...l.net>, <dri-devel@...ts.freedesktop.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC:     <mparab@...ence.com>, <yamonkar@...ence.com>, <jsarha@...com>,
        <nsekhar@...com>, <praneeth@...com>, <nikhil.nd@...com>
Subject: Re: [PATCH v9 2/3] drm: bridge: Add support for Cadence MHDP8546
 DPI/DP bridge

Hi Swapnil,

On 31/08/2020 11:23, Swapnil Jakhade wrote:

> +	line_thresh1 = ((vs + 1) << 5) * 8 / bpp;
> +	line_thresh2 = (pxlclock << 5) / 1000 / rate * (vs + 1) - (1 << 5);
> +	line_thresh = line_thresh1 - line_thresh2 / mhdp->link.num_lanes;
> +	line_thresh = (line_thresh >> 5) + 2;

These calculations do not seem to go correctly. There's no comment what's the logic here, but e.g.
for 640x480 (pxlclock 31500) with 1.62Gbps link, I get vs=4, and then the second line above comes to:

(31500 << 5) / 1000 / 162 * (4+1) - (1<<5) = -0.8888888888888893

The result is line_thresh of 100663299.

 Tomi

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