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

[Maxime Ripard <maxime.ripard@bootlin.com>]

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On Thu, Mar 21, 2019 at 07:40:32PM +0530, Jagan Teki wrote:
> On Tue, Mar 19, 2019 at 4:23 PM Maxime Ripard <maxime.ripard@bootlin.com> wrote:
> >
> > 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@bootlin.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.
>
> I have added it in 3rd paragraph in commit message, may be you missed
> it or you may look for different text.
>
> "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."

And you're still not explaining *why* that is an issue, and what makes
you think that your solution is the right one.

Seriously, I feel like this discussion is going in circles. I've been
asking for that since your very first version.

> > > 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
>
> Yes, I have replied at that time itself, please check.
>
> http://lists.infradead.org/pipermail/linux-arm-kernel/2019-February/629673.html
>
> Here I have taken the pixel clock to 30Mhz for example in Bananapi
> panel, and the above gist the pixel is 148Mhz from another panel.

Again, without providing any reference, just making vague statements.

Maxime

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

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