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

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Message-ID: <4c740b30-9ba0-c5a9-13b6-7f6c3b417595@denx.de>
Date:   Mon, 23 May 2022 15:25:51 +0200
From:   Marek Vasut <marex@...x.de>
To:     Jonathan Liu <net147@...il.com>
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

On 5/23/22 15:20, Jonathan Liu wrote:
> 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.

Doh, you're right.

Reviewed-by: Marek Vasut <marex@...x.de>