linux-kernel - Re: [PATCH V1 2/2] backlight: qcom-wled: Correct the sync

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Message-ID: <20210224112632.lgm2yj6ekayuqg2p@maple.lan>
Date:   Wed, 24 Feb 2021 11:26:32 +0000
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Kiran Gunda <kgunda@...eaurora.org>
Cc:     bjorn.andersson@...aro.org, jingoohan1@...il.com,
        lee.jones@...aro.org, b.zolnierkie@...sung.com,
        dri-devel@...ts.freedesktop.org, jacek.anaszewski@...il.com,
        pavel@....cz, robh+dt@...nel.org, mark.rutland@....com,
        linux-leds@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Andy Gross <agross@...nel.org>,
        linux-arm-msm@...r.kernel.org, linux-fbdev@...r.kernel.org,
        phone-devel@...r.kernel.org
Subject: Re: [PATCH V1 2/2] backlight: qcom-wled: Correct the sync_toggle
 sequence

On Wed, Feb 24, 2021 at 09:20:48AM +0530, Kiran Gunda wrote:
> Currently the FSC SYNC_BIT and MOD_SYNC_BIT are toggled
> from 1 to 0 to update the FSC and brightenss settings.
> Change this sequence form 0 to 1 as per the hardware team
> recommendation to update the FSC and brightness correctly.

Again... this patch description feels somewhat rushed. A patch
description is there to support code reviewer and to go on the version
history to assist with future maintainance. They matter!

Anyhow I don't recognise the "from 1 to 0" in the code since both before
an after the change it goes "from 0 to 1" and "from 1 to 0" but in a
different order. Doesn't the code actually currently implement "set then
clear"? If so then, likewise the new code is adopting "clear then set".

As with patch 1, the sync bits modified by wled3_sync_toggle singular
or plural?

Finally a description that is more sympathetic to the reviewer would be
welcome.  For example the following (if my guess is right and it is
true) makes things much easier for the reviewer:

  "The 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".
 

Daniel.


> 
> Signed-off-by: Kiran Gunda <kgunda@...eaurora.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
>