linux-kernel - Re: [PATCH v2 2/4] drm/vc4: hdmi: Add pixel bvb clock control

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Message-ID: <da50b700-2566-2b5e-766f-77621405c8e3@samsung.com>
Date:   Wed, 2 Sep 2020 12:13:44 +0900
From:   Hoegeun Kwon <hoegeun.kwon@...sung.com>
To:     Chanwoo Choi <cw00.choi@...sung.com>, nsaenzjulienne@...e.de,
        eric@...olt.net, maxime@...no.tech, stefan.wahren@...e.com,
        dave.stevenson@...pberrypi.com
Cc:     devicetree@...r.kernel.org, tim.gover@...pberrypi.com,
        sboyd@...nel.org, mturquette@...libre.com, kdasu.kdev@...il.com,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-clk@...r.kernel.org, robh+dt@...nel.org,
        bcm-kernel-feedback-list@...adcom.com,
        linux-rpi-kernel@...ts.infradead.org, phil@...pberrypi.com,
        linux-arm-kernel@...ts.infradead.org,
        Hoegeun Kwon <hoegeun.kwon@...sung.com>
Subject: Re: [PATCH v2 2/4] drm/vc4: hdmi: Add pixel bvb clock control

Hi Chanwoo,

On 9/1/20 1:27 PM, Chanwoo Choi wrote:
> Hi Hoegeun,
>
> It looks good to me. But, just one comment.
>
> On 9/1/20 1:07 PM, Hoegeun Kwon wrote:
>> There is a problem that the output does not work at a resolution
>> exceeding FHD. To solve this, we need to adjust the bvb clock at a
>> resolution exceeding FHD.
>>
>> Signed-off-by: Hoegeun Kwon <hoegeun.kwon@...sung.com>
>> ---
>>   drivers/gpu/drm/vc4/vc4_hdmi.c | 25 +++++++++++++++++++++++++
>>   drivers/gpu/drm/vc4/vc4_hdmi.h |  1 +
>>   2 files changed, 26 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
>> index 95ec5eedea39..eb3192d1fd86 100644
>> --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
>> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
>> @@ -80,6 +80,7 @@
>>   # define VC4_HD_M_ENABLE			BIT(0)
>>   
>>   #define CEC_CLOCK_FREQ 40000
>> +#define VC4_HSM_MID_CLOCK 149985000
>>   
>>   static int vc4_hdmi_debugfs_regs(struct seq_file *m, void *unused)
>>   {
>> @@ -380,6 +381,7 @@ static void vc4_hdmi_encoder_post_crtc_powerdown(struct drm_encoder *encoder)
>>   	HDMI_WRITE(HDMI_VID_CTL,
>>   		   HDMI_READ(HDMI_VID_CTL) & ~VC4_HD_VID_CTL_ENABLE);
>>   
>> +	clk_disable_unprepare(vc4_hdmi->pixel_bvb_clock);
>>   	clk_disable_unprepare(vc4_hdmi->hsm_clock);
>>   	clk_disable_unprepare(vc4_hdmi->pixel_clock);
>>   
>> @@ -638,6 +640,23 @@ static void vc4_hdmi_encoder_pre_crtc_configure(struct drm_encoder *encoder)
>>   		return;
>>   	}
>>   
>> +	ret = clk_set_rate(vc4_hdmi->pixel_bvb_clock,
>> +			(hsm_rate > VC4_HSM_MID_CLOCK ? 150000000 : 75000000));
>> +	if (ret) {
>> +		DRM_ERROR("Failed to set pixel bvb clock rate: %d\n", ret);
>> +		clk_disable_unprepare(vc4_hdmi->hsm_clock);
>> +		clk_disable_unprepare(vc4_hdmi->pixel_clock);
>> +		return;
>> +	}
>> +
>> +	ret = clk_prepare_enable(vc4_hdmi->pixel_bvb_clock);
>> +	if (ret) {
>> +		DRM_ERROR("Failed to turn on pixel bvb clock: %d\n", ret);
>> +		clk_disable_unprepare(vc4_hdmi->hsm_clock);
>> +		clk_disable_unprepare(vc4_hdmi->pixel_clock);
>> +		return;
>> +	}
> Generally, enable the clock before using clk and then change the clock rate.
> I think that you better to change the order between clk_prepare_enable and clk_set_rate.

Thank you for your comment.


As Maxime answered in another patch [1], there is no clear rule of order 
here.

[1] https://lkml.org/lkml/2020/9/1/327


Best regards,

Hoegeun