linux-kernel - Re: [PATCH 1/7] drm/rockchip/dsi: correct Feedback divider setting

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Message-ID: <20170919181751.GA38656@google.com>
Date:   Tue, 19 Sep 2017 11:19:01 -0700
From:   Brian Norris <briannorris@...omium.org>
To:     Sean Paul <seanpaul@...omium.org>
Cc:     Nickey Yang <nickey.yang@...k-chips.com>, mark.yao@...k-chips.com,
        robh+dt@...nel.org, heiko@...ech.de, mark.rutland@....com,
        airlied@...ux.ie, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        linux-rockchip@...ts.infradead.org, hl@...k-chips.com,
        zyw@...k-chips.com, bivvy.bi@...k-chips.com, xbl@...k-chips.com
Subject: Re: [PATCH 1/7] drm/rockchip/dsi: correct Feedback divider setting

Hi Sean,

On Tue, Sep 19, 2017 at 11:00:25AM -0700, Sean Paul wrote:
> On Mon, Sep 18, 2017 at 05:05:33PM +0800, Nickey Yang wrote:
> > This patch correct Feedback divider setting:
> > 1、Set Feedback divider [8:5] when HIGH_PROGRAM_EN
> > 2、Due to the use of a "by 2 pre-scaler," the range of the
> > feedback multiplication Feedback divider is limited to even
> > division numbers, and Feedback divider must be greater than
> > 12, less than 1000.
> > 3、Make the previously configured Feedback divider(LSB)
> > factors effective
> > 
> > Signed-off-by: Nickey Yang <nickey.yang@...k-chips.com>
> > ---
> >  drivers/gpu/drm/rockchip/dw-mipi-dsi.c | 83 ++++++++++++++++++++++------------
> >  1 file changed, 54 insertions(+), 29 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> > index 9a20b9d..52698b7 100644
> > --- a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> > +++ b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> > @@ -228,7 +228,7 @@
> >  #define LOW_PROGRAM_EN		0
> >  #define HIGH_PROGRAM_EN		BIT(7)
> >  #define LOOP_DIV_LOW_SEL(val)	(((val) - 1) & 0x1f)
> > -#define LOOP_DIV_HIGH_SEL(val)	((((val) - 1) >> 5) & 0x1f)
> > +#define LOOP_DIV_HIGH_SEL(val)	((((val) - 1) >> 5) & 0xf)
> >  #define PLL_LOOP_DIV_EN		BIT(5)
> >  #define PLL_INPUT_DIV_EN	BIT(4)
> >  
> > @@ -461,6 +461,7 @@ static int dw_mipi_dsi_phy_init(struct dw_mipi_dsi *dsi)
> >  	dw_mipi_dsi_phy_write(dsi, 0x17, INPUT_DIVIDER(dsi->input_div));
> >  	dw_mipi_dsi_phy_write(dsi, 0x18, LOOP_DIV_LOW_SEL(dsi->feedback_div) |
> >  					 LOW_PROGRAM_EN);
> > +	dw_mipi_dsi_phy_write(dsi, 0x19, PLL_LOOP_DIV_EN | PLL_INPUT_DIV_EN);
> 
> You do the same write 2 lines down. Are both needed? It would be nice if the
> register names were also defined, so this is easier to read.

If I'm reading correctly, I think this is what Nickey meant by:

"3、Make the previously configured Feedback divider(LSB)
factors effective"

. My reading of the databook is that this step finalizes the previous
two writes (to test code 0x17 and 0x18).

Given this was buggy (?) previously, it does seem like having some extra
language to document this could help. Register names (or "test codes",
per the docs?) could help, but additionally, maybe a few more comments.

> >  	dw_mipi_dsi_phy_write(dsi, 0x18, LOOP_DIV_HIGH_SEL(dsi->feedback_div) |
> >  					 HIGH_PROGRAM_EN);
> >  	dw_mipi_dsi_phy_write(dsi, 0x19, PLL_LOOP_DIV_EN | PLL_INPUT_DIV_EN);

[...]

Brian