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Subject: Re: [v2] drm/sun4i: add lvds mode_valid function
To:     Chen-Yu Tsai <wens@csie.org>,
        Maxime Ripard <maxime.ripard@free-electrons.com>,
        David Airlie <airlied@linux.ie>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
References: <1520940019-68977-1-git-send-email-giulio.benetti@micronovasrl.com>
 <20180419133421.6fx3whge7h364vd3@core>
 <CAGb2v64dqPoJkJDuMWp66HYXvZ3RJkrRz+2-CUVEcEG1k7UTsw@mail.gmail.com>
From:   Giulio Benetti <giulio.benetti@micronovasrl.com>
Message-ID: <129dfeb0-0962-608d-370e-277a3acce7ca@micronovasrl.com>
Date:   Thu, 19 Apr 2018 16:02:08 +0200
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Hi everybody,

Il 19/04/2018 15:36, Chen-Yu Tsai ha scritto:
> On Thu, Apr 19, 2018 at 9:34 PM, Ondřej Jirman
> <doudahwezomiechahtah@xff.cz> wrote:
>> Hello Giulio,
>>
>> this patch breaks LVDS output on A83T. Without it, modesetting works,
>> with it there's no output.
>>
>> Some more info below...
>>
>> On Tue, Mar 13, 2018 at 12:20:19PM +0100, Giulio Benetti wrote:
>>> mode_valid function is missing for lvds.
>>>
>>> Add it making it pointed by encoder helper functions.
>>>
>>> Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
>>> ---
>>>   drivers/gpu/drm/sun4i/sun4i_lvds.c | 55 ++++++++++++++++++++++++++++++++++++++
>>>   1 file changed, 55 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/sun4i/sun4i_lvds.c b/drivers/gpu/drm/sun4i/sun4i_lvds.c
>>> index be3f14d..bffff4c 100644
>>> --- a/drivers/gpu/drm/sun4i/sun4i_lvds.c
>>> +++ b/drivers/gpu/drm/sun4i/sun4i_lvds.c
>>> @@ -94,9 +94,64 @@ static void sun4i_lvds_encoder_disable(struct drm_encoder *encoder)
>>>        }
>>>   }
>>>
>>> +static enum drm_mode_status sun4i_lvds_encoder_mode_valid(struct drm_encoder *crtc,
>>> +                                                       const struct drm_display_mode *mode)
>>> +{
>>> +     struct sun4i_lvds *lvds = drm_encoder_to_sun4i_lvds(crtc);
>>> +     struct sun4i_tcon *tcon = lvds->tcon;
>>> +     u32 hsync = mode->hsync_end - mode->hsync_start;
>>> +     u32 vsync = mode->vsync_end - mode->vsync_start;
>>> +     unsigned long rate = mode->clock * 1000;
>>> +     long rounded_rate;
>>> +
>>> +     DRM_DEBUG_DRIVER("Validating modes...\n");
>>> +
>>> +     if (hsync < 1)
>>> +             return MODE_HSYNC_NARROW;
>>> +
>>> +     if (hsync > 0x3ff)
>>> +             return MODE_HSYNC_WIDE;
>>> +
>>> +     if ((mode->hdisplay < 1) || (mode->htotal < 1))
>>> +             return MODE_H_ILLEGAL;
>>> +
>>> +     if ((mode->hdisplay > 0x7ff) || (mode->htotal > 0xfff))
>>> +             return MODE_BAD_HVALUE;
>>> +
>>> +     DRM_DEBUG_DRIVER("Horizontal parameters OK\n");
>>> +
>>> +     if (vsync < 1)
>>> +             return MODE_VSYNC_NARROW;
>>> +
>>> +     if (vsync > 0x3ff)
>>> +             return MODE_VSYNC_WIDE;
>>> +
>>> +     if ((mode->vdisplay < 1) || (mode->vtotal < 1))
>>> +             return MODE_V_ILLEGAL;
>>> +
>>> +     if ((mode->vdisplay > 0x7ff) || (mode->vtotal > 0xfff))
>>> +             return MODE_BAD_VVALUE;
>>> +
>>> +     DRM_DEBUG_DRIVER("Vertical parameters OK\n");
>>> +
>>> +     tcon->dclk_min_div = 7;
>>> +     tcon->dclk_max_div = 7;
>>
>> Why would validation function change any state anywhere?
>>
>>> +     rounded_rate = clk_round_rate(tcon->dclk, rate);
>>
>> The issue is here, on A83T TBS A711 tablet, I get...
>>
>> sun4i-tcon 1c0c000.lcd-controller: XXX: hsync=20 hdisplay=1024 htotal=1384
>>    vsync=5 vdisplay=600 vtotal=640 rate=52000000 rounded_rate=51857142
>>
>>> +     if (rounded_rate < rate)
>>> +             return MODE_CLOCK_LOW;
>>> +
>>> +     if (rounded_rate > rate)
>>> +             return MODE_CLOCK_HIGH;
>>
>> ... while the previous conditions require an exact match for some reason.
>>
>> But HW works fine without an exact rate match. Why is exact match required here?
> 
> This thread might provide some more info, though we could never get an
> agreement.
> 
> https://patchwork.kernel.org/patch/9446385/

Thanks for pointing that Thread ChenYu.
So the only chance is to trim frequency according to encoder capabilities.
I agree to block when encoder can't provide frequency specified,
otherwise you could drive you panel at the near the lowest(highest)
frequency and get out of limits for a few without knowing it.

Best regards

-- 
Giulio Benetti
CTO

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> 
> ChenYu
> 
>>
>> thank you,
>>    Ondrej
>>
>>> +     DRM_DEBUG_DRIVER("Clock rate OK\n");
>>> +
>>> +     return MODE_OK;
>>> +}
>>> +
>>>   static const struct drm_encoder_helper_funcs sun4i_lvds_enc_helper_funcs = {
>>>        .disable        = sun4i_lvds_encoder_disable,
>>>        .enable         = sun4i_lvds_encoder_enable,
>>> +     .mode_valid     = sun4i_lvds_encoder_mode_valid,
>>>   };
>>>
>>>   static const struct drm_encoder_funcs sun4i_lvds_enc_funcs = {
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