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

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Message-ID: <35622307-5e88-a2ed-bdf9-fca6554efefc@benettiengineering.com>
Date:   Fri, 8 Jan 2021 15:34:52 +0100
From:   Giulio Benetti <giulio.benetti@...ettiengineering.com>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Marjan Pascolo <marjan.pascolo@...xom.it>, 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,

On 1/8/21 10:23 AM, Maxime Ripard wrote:
> 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 :)

Ok, didn't know about this tag

>> 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?

The idea was to have 2 for testing, 1st one is already applicable, while 
the other must be tested, but I can send only one with no problem.

> That patch only seem to undo what you did in patch 1

No, it doesn't, the 2nd one change the way it achieve the same thing, 
because the 1st swap DCLK phase, while the 2nd uses the IO_POL bit to 
set IO polarity according to bus_flags.

Best Regards
-- 
Giulio Benetti
Benetti Engineering sas