linux-kernel - Re: [PATCH v2 2/2] drm/sun4i: tcon: improve DCLK polarity handling

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Message-ID: <88789919-6f46-55f2-d432-d83c91b6e73f@benettiengineering.com>
Date:   Fri, 8 Jan 2021 15:46:15 +0100
From:   Giulio Benetti <giulio.benetti@...ettiengineering.com>
To:     Marjan Pascolo <marjan.pascolo@...xom.it>,
        Maxime Ripard <maxime@...no.tech>
Cc:     wens@...e.org, daniel@...ll.ch, airlied@...ux.ie,
        treding@...dia.com, Jernej Skrabec <jernej.skrabec@...l.net>,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Giulio Benetti <giulio.benetti@...ronovasrl.com>
Subject: Re: [PATCH v2 2/2] drm/sun4i: tcon: improve DCLK polarity handling

Hi Marjan,

On 1/8/21 10:32 AM, Marjan Pascolo wrote:
> Hi,

please don't top post, answer in-line as we do, and please use 
plain-test instead of HTML. These are the standards in Mailing Lists(ML).

> Quote "
> 
> I'm not really sure why we need the first patch of this series here?
> That patch only seem to undo what you did in patch 1
> 
> "

Already answered to Maxime

> 
> And another question (probably could be a stupid one):
> 
> in "/[PATCH v2 2/2] drm/sun4i: tcon: improve DCLK polarity handling/" I 
> see you deleted:
> 
> -		clk_set_phase(tcon->dclk, 0);
> 
> Is safe to assume that phase register will be always set to 0?

We always assumed it is set to 0 by default.

> 
> Or maybe will be safer manually set it to 0 in every condition to avoid 
> surprises (dirt values due to previous condition)?

That would be a good idea, so something like this:

'''
	int phase = 0;

	if (info->bus_flags & DRM_BUS_FLAG_PIXDATA_DRIVE_NEGEDGE)
		phase = 240;

	clk_set_phase(tcon->dclk, phase);
'''

because now DRM_BUS_FLAG_PIXDATA_SAMPLE_ and DRM_BUS_FLAG_PIXDATA_DRIVE_ 
are exclusive, while before not.

But then if bit 26 solution works everything gets easier.

Best Regards
Giulio

> 
> Marjan
> 
> Il 08/01/2021 10:23, Maxime Ripard ha scritto:
>> Hi,
>>
>> Thanks for those patches
>>
>> On Thu, Jan 07, 2021 at 03:30:32AM +0100, Giulio Benetti wrote:
>>> From: Giulio Benetti<giulio.benetti@...ronovasrl.com>
>>>
>>> It turned out(Maxime suggestion) that bit 26 of SUN4I_TCON0_IO_POL_REG is
>>> dedicated to invert DCLK polarity and this makes thing really easier than
>>> before. So let's handle DCLK polarity by adding
>>> SUN4I_TCON0_IO_POL_DCLK_POSITIVE as bit 26 and activating according to
>>> bus_flags the same way is done for all the other signals.
>>>
>>> Cc: Maxime Ripard<maxime@...no.tech>
>> Suggested-by would be nice here :)
>>
>>> Signed-off-by: Giulio Benetti<giulio.benetti@...ronovasrl.com>
>>> ---
>>>   drivers/gpu/drm/sun4i/sun4i_tcon.c | 20 +-------------------
>>>   drivers/gpu/drm/sun4i/sun4i_tcon.h |  1 +
>>>   2 files changed, 2 insertions(+), 19 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/sun4i/sun4i_tcon.c b/drivers/gpu/drm/sun4i/sun4i_tcon.c
>>> index 52598bb0fb0b..30171ccd87e5 100644
>>> --- a/drivers/gpu/drm/sun4i/sun4i_tcon.c
>>> +++ b/drivers/gpu/drm/sun4i/sun4i_tcon.c
>>> @@ -569,26 +569,8 @@ static void sun4i_tcon0_mode_set_rgb(struct sun4i_tcon *tcon,
>>>   	if (info->bus_flags & DRM_BUS_FLAG_DE_LOW)
>>>   		val |= SUN4I_TCON0_IO_POL_DE_NEGATIVE;
>>>   
>>> -	/*
>>> -	 * On A20 and similar SoCs, the only way to achieve Positive Edge
>>> -	 * (Rising Edge), is setting dclk clock phase to 2/3(240°).
>>> -	 * By default TCON works in Negative Edge(Falling Edge),
>>> -	 * this is why phase is set to 0 in that case.
>>> -	 * Unfortunately there's no way to logically invert dclk through
>>> -	 * IO_POL register.
>>> -	 * The only acceptable way to work, triple checked with scope,
>>> -	 * is using clock phase set to 0° for Negative Edge and set to 240°
>>> -	 * for Positive Edge.
>>> -	 * On A33 and similar SoCs there would be a 90° phase option,
>>> -	 * but it divides also dclk by 2.
>>> -	 * Following code is a way to avoid quirks all around TCON
>>> -	 * and DOTCLOCK drivers.
>>> -	 */
>>>   	if (info->bus_flags & DRM_BUS_FLAG_PIXDATA_DRIVE_POSEDGE)
>>> -		clk_set_phase(tcon->dclk, 0);
>>> -
>>> -	if (info->bus_flags & DRM_BUS_FLAG_PIXDATA_DRIVE_NEGEDGE)
>>> -		clk_set_phase(tcon->dclk, 240);
>>> +		val |= SUN4I_TCON0_IO_POL_DCLK_POSITIVE;
>> I'm not really sure why we need the first patch of this series here?
>> That patch only seem to undo what you did in patch 1
>>
>> Maxime