linux-kernel - Re: [PATCH v8 02/15] drm/sun4i: tcon: Compute DCLK dividers based on format, lanes

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Message-ID: <20190319105307.omowzyhqu33e3pzy@flea>
Date:   Tue, 19 Mar 2019 11:53:07 +0100
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Jagan Teki <jagan@...rulasolutions.com>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        Chen-Yu Tsai <wens@...e.org>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        devicetree <devicetree@...r.kernel.org>,
        Michael Trimarchi <michael@...rulasolutions.com>,
        linux-amarula@...rulasolutions.com,
        linux-sunxi <linux-sunxi@...glegroups.com>
Subject: Re: [PATCH v8 02/15] drm/sun4i: tcon: Compute DCLK dividers based on
 format, lanes

On Mon, Mar 11, 2019 at 09:36:27PM +0530, Jagan Teki wrote:
> On Mon, Mar 11, 2019 at 9:08 PM Maxime Ripard <maxime.ripard@...tlin.com> wrote:
> >
> > On Mon, Mar 11, 2019 at 07:06:24PM +0530, Jagan Teki wrote:
> > > pll-video => pll-mipi => tcon0 => tcon0-pixel-clock is the typical
> > > MIPI clock topology in Allwinner DSI controller.
> > >
> > > TCON dotclock driver is computing the desired DCLK divider based on
> > > panel pixel clock along with input DCLK min, max divider values from
> > > tcon driver and that would eventually set the pll-mipi clock rate.
> > >
> > > The current code allows the TCON clock divider to have a default 4
> > > for min, max ranges that would fail to compute the desired pll-mipi
> > > rate while supporting new panels.
> > >
> > > So, add the computation logic 'format/lanes' to dclk min and max dividers
> > > and instead of default 4. This computation logic align with Allwinner A64
> > > BSP, hoping that would work even for A33.
> >
> > Last time we discussed this, we found out that this wasn't the case,
> > even in the BSP.
>
> This was the case for BSP to compute pll-mipi not for TCON_DSI clock
> register, SUN4I_TCON0_DCLK_REG, which marked the divider 4 by default.
>
> > What compelling evidence have you found that makes you say otherwise?
>
> divider 4 isn't worked, this I would mentioned before as well.

Maybe you mentionned it before, but it's nowhere to be found in your
commit log.

> Tested this on 4 different panels, and below are the desired divider values
> and pll-mipi clock rate with respect to pixel clock frequency.
>
> - 55MHz pixel clock with 4-lane panel, and the desired DSI clock divider
>   is 6 with the output parent clock rate of 330MHz.
> - 30MHz pixel clock with 4-lane panel, and the desired DSI clock divider
>   is 6 with parent clock rate of 180MHz.
> - 27.5Mhz pixel clock with 2-lane pane, and the desired DSI clock divider
>   is 12 with the output parent clock rate of 330MHz.
> - 147MHz pixel clock with 4-lane panel, and the desired DSI clock divider
>   is 6 with the output parent clock rate of 882MHz.
>
> BSP trying to use this format/lane to compute dsi divider that in-turn
> using pll-mipi set_rate but TCON0_DCLK_REG keep constant 4.

Feel free to reply to
http://lists.infradead.org/pipermail/linux-arm-kernel/2019-February/629596.html

And correct whatever is said there that isn't what is happening.

Maxime

--
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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