Re: [PATCH V2 2/2] backlight: qcom-wled: Correct the sync_toggle sequence

From: kgunda@codeaurora.org
To: Daniel Thompson <daniel.thompson@linaro.org>
Cc: bjorn.andersson@linaro.org, jingoohan1@gmail.com,
	lee.jones@linaro.org, b.zolnierkie@samsung.com,
	dri-devel@lists.freedesktop.org, jacek.anaszewski@gmail.com,
	pavel@ucw.cz, robh+dt@kernel.org, mark.rutland@arm.com,
	linux-leds@vger.kernel.org, devicetree@vger.kernel.org,
	linux-kernel@vger.kernel.org, Andy Gross <agross@kernel.org>,
	linux-arm-msm@vger.kernel.org, linux-fbdev@vger.kernel.org,
	phone-devel@vger.kernel.org
Subject: Re: [PATCH V2 2/2] backlight: qcom-wled: Correct the sync_toggle sequence
Date: Mon, 01 Mar 2021 14:15:12 +0530	[thread overview]
Message-ID: <0cca377c2a7648c5f1606e38ba1b7d4d@codeaurora.org> (raw)
In-Reply-To: <20210226172601.aknj2d4hghkkqjol@maple.lan>

On 2021-02-26 22:56, Daniel Thompson wrote:
> On Fri, Feb 26, 2021 at 05:42:24PM +0530, Kiran Gunda wrote:
>> As per the current implementation, after FSC (Full Scale Current)
>> and brightness update the sync bits are transitioned from 1 to 0.
> 
> This still seems to incorrectly describe the current behaviour.
> 
> Surely in most cases (i.e. every time except the first) the value of 
> the
> sync bit is 0 when the function is called and we get both a 0 to 1
> and then a 1 to 0 transition.
> 
> That is why I recommended set-then-clear terminology to describe the
> current behaviour. It is concise and correct.
> 
> 
> Daniel.
> 
> 
> 
Okay. Actually I have mentioned the "clear-and-set" in explaining the 
fix.
Let me modify the same terminology in explaining the problem case also.

>> But, the FSC and brightness sync takes place during a 0 to 1
>> transition of the sync bits. So the hardware team recommends a
>> clear-then-set approach in order to guarantee such a transition
>> regardless of the previous register state.
>> 
>> Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>
>> ---
>>  drivers/video/backlight/qcom-wled.c | 12 ++++++------
>>  1 file changed, 6 insertions(+), 6 deletions(-)
>> 
>> diff --git a/drivers/video/backlight/qcom-wled.c 
>> b/drivers/video/backlight/qcom-wled.c
>> index aef52b9..19f83ac 100644
>> --- a/drivers/video/backlight/qcom-wled.c
>> +++ b/drivers/video/backlight/qcom-wled.c
>> @@ -337,13 +337,13 @@ static int wled3_sync_toggle(struct wled *wled)
>> 
>>  	rc = regmap_update_bits(wled->regmap,
>>  				wled->ctrl_addr + WLED3_SINK_REG_SYNC,
>> -				mask, mask);
>> +				mask, WLED3_SINK_REG_SYNC_CLEAR);
>>  	if (rc < 0)
>>  		return rc;
>> 
>>  	rc = regmap_update_bits(wled->regmap,
>>  				wled->ctrl_addr + WLED3_SINK_REG_SYNC,
>> -				mask, WLED3_SINK_REG_SYNC_CLEAR);
>> +				mask, mask);
>> 
>>  	return rc;
>>  }
>> @@ -353,17 +353,17 @@ static int wled5_mod_sync_toggle(struct wled 
>> *wled)
>>  	int rc;
>>  	u8 val;
>> 
>> -	val = (wled->cfg.mod_sel == MOD_A) ? WLED5_SINK_REG_SYNC_MOD_A_BIT :
>> -					     WLED5_SINK_REG_SYNC_MOD_B_BIT;
>>  	rc = regmap_update_bits(wled->regmap,
>>  				wled->sink_addr + WLED5_SINK_REG_MOD_SYNC_BIT,
>> -				WLED5_SINK_REG_SYNC_MASK, val);
>> +				WLED5_SINK_REG_SYNC_MASK, 0);
>>  	if (rc < 0)
>>  		return rc;
>> 
>> +	val = (wled->cfg.mod_sel == MOD_A) ? WLED5_SINK_REG_SYNC_MOD_A_BIT :
>> +					     WLED5_SINK_REG_SYNC_MOD_B_BIT;
>>  	return regmap_update_bits(wled->regmap,
>>  				  wled->sink_addr + WLED5_SINK_REG_MOD_SYNC_BIT,
>> -				  WLED5_SINK_REG_SYNC_MASK, 0);
>> +				  WLED5_SINK_REG_SYNC_MASK, val);
>>  }
>> 
>>  static int wled_ovp_fault_status(struct wled *wled, bool *fault_set)
>> --
>> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora 
>> Forum,
>>  a Linux Foundation Collaborative Project
>>