linux-kernel - Re: [PATCH v2] drm: bridge: icn6211: Adjust clock phase using SYS_CTRL

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Date:   Mon, 23 May 2022 23:20:15 +1000
From:   Jonathan Liu <net147@...il.com>
To:     Marek Vasut <marex@...x.de>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Maxime Ripard <maxime@...no.tech>,
        Jagan Teki <jagan@...rulasolutions.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Robert Foss <robert.foss@...aro.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] drm: bridge: icn6211: Adjust clock phase using SYS_CTRL_1

Hi Marek,

On Mon, 23 May 2022 at 23:15, Marek Vasut <marex@...x.de> wrote:
>
> On 5/23/22 15:01, Jonathan Liu wrote:
> > The code from [1] sets SYS_CTRL_1 to different values depending on the
> > desired clock phase (0, 1/4, 1/2 or 3/4). A clock phase of 0 aligns the
> > positive edge of the clock with the pixel data while other values delay
> > the clock by a fraction of the clock period. A clock phase of 1/2 aligns
> > the negative edge of the clock with the pixel data.
> >
> > The driver currently hard codes SYS_CTRL_1 to 0x88 which corresponds to
> > aligning the positive edge of the clock with the pixel data. This won't
> > work correctly for panels that require aligning the negative edge of the
> > clock with the pixel data.
> >
> > Adjust the clock phase to 0 if DRM_BUS_FLAG_PIXDATA_DRIVE_POSEDGE is
> > present in bus_flags, otherwise adjust the clock phase to 1/2 as
> > appropriate for DRM_BUS_FLAG_PIXDATA_DRIVE_NEGEDGE.
> >
> > [1] https://github.com/tdjastrzebski/ICN6211-Configurator
> >
> > Signed-off-by: Jonathan Liu <net147@...il.com>
> > ---
> > V2: Use GENMASK and FIELD_PREP macros
> > ---
> >   drivers/gpu/drm/bridge/chipone-icn6211.c | 18 ++++++++++++++++--
> >   1 file changed, 16 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/bridge/chipone-icn6211.c b/drivers/gpu/drm/bridge/chipone-icn6211.c
> > index 47dea657a752..f1538fb5f8a9 100644
> > --- a/drivers/gpu/drm/bridge/chipone-icn6211.c
> > +++ b/drivers/gpu/drm/bridge/chipone-icn6211.c
> > @@ -9,6 +9,8 @@
> >   #include <drm/drm_print.h>
> >   #include <drm/drm_mipi_dsi.h>
> >
> > +#include <linux/bitfield.h>
> > +#include <linux/bits.h>
> >   #include <linux/delay.h>
> >   #include <linux/gpio/consumer.h>
> >   #include <linux/i2c.h>
> > @@ -26,6 +28,11 @@
> >   #define PD_CTRL(n)          (0x0a + ((n) & 0x3)) /* 0..3 */
> >   #define RST_CTRL(n)         (0x0e + ((n) & 0x1)) /* 0..1 */
> >   #define SYS_CTRL(n)         (0x10 + ((n) & 0x7)) /* 0..4 */
> > +#define SYS_CTRL_1_CLK_PHASE_MSK     GENMASK(5, 4)
>
> This should be GENMASK(7, 6) , no ?

Clock phase 0 = 0b_1000_1000 = 0x88
Clock phase 1/4 = 0b_1001_1000 = 0x98
Clock phase 1/2 = 0b_1010_1000 = 0xA8
Clock phase 3/4 = 0b_1011_1000 = 0xB8

The clock phase bits are 5:4 not 7:6. The upper 2 bits and lower 4
bits are unknown.

Regards,
Jonathan