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

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Date:   Tue, 19 Sep 2017 13:27:40 -0700
From:   Sean Paul <seanpaul@...omium.org>
To:     Brian Norris <briannorris@...omium.org>
Cc:     Sean Paul <seanpaul@...omium.org>,
        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

On Tue, Sep 19, 2017 at 11:19:01AM -0700, Brian Norris wrote:
> 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.
> 

Ah, yeah, thanks for the explanation. It's not clear that this latches the 
values above. I think register names and comments would be immensely helpful.

Sean

> > >  	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

-- 
Sean Paul, Software Engineer, Google / Chromium OS