linux-kernel - Re: [PATCH] drm/msm/dp: fix the max supported bpp logic

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Message-ID: <8fa86c0c-183b-4787-9525-38dfe6bcecc6@quicinc.com>
Date: Mon, 29 Jul 2024 11:28:35 -0700
From: Abhinav Kumar <quic_abhinavk@...cinc.com>
To: Stephen Boyd <swboyd@...omium.org>, Daniel Vetter <daniel@...ll.ch>,
        "David Airlie" <airlied@...il.com>,
        Dmitry Baryshkov
	<dmitry.baryshkov@...aro.org>,
        Guenter Roeck <groeck@...omium.org>,
        Marijn
 Suijten <marijn.suijten@...ainline.org>,
        Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>,
        Tanmay Shah <tanmay@...eaurora.org>,
        Vara Reddy
	<quic_varar@...cinc.com>,
        <freedreno@...ts.freedesktop.org>
CC: <dri-devel@...ts.freedesktop.org>, <quic_jesszhan@...cinc.com>,
        <neil.armstrong@...aro.org>, <abel.vesa@...aro.org>,
        <quic_khsieh@...cinc.com>, Rob Clark <robdclark@...omium.org>,
        "Chandan
 Uddaraju" <chandanu@...eaurora.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/msm/dp: fix the max supported bpp logic

Hi Stephen

On 7/26/2024 5:24 PM, Stephen Boyd wrote:
> Quoting Abhinav Kumar (2024-07-25 15:03:19)
>> diff --git a/drivers/gpu/drm/msm/dp/dp_panel.c b/drivers/gpu/drm/msm/dp/dp_panel.c
>> index a916b5f3b317..56ce5e4008f8 100644
>> --- a/drivers/gpu/drm/msm/dp/dp_panel.c
>> +++ b/drivers/gpu/drm/msm/dp/dp_panel.c
>> @@ -423,8 +424,10 @@ int dp_panel_init_panel_info(struct dp_panel *dp_panel)
>>                                  drm_mode->clock);
>>          drm_dbg_dp(panel->drm_dev, "bpp = %d\n", dp_panel->dp_mode.bpp);
>>
>> -       dp_panel->dp_mode.bpp = max_t(u32, 18,
>> -                               min_t(u32, dp_panel->dp_mode.bpp, 30));
>> +       max_supported_bpp = dp_panel_get_mode_bpp(dp_panel, dp_panel->dp_mode.bpp,
>> +                                                 dp_panel->dp_mode.drm_mode.clock);
>> +       dp_panel->dp_mode.bpp = max_t(u32, 18, max_supported_bpp);
> 
> Is the max_t() usage still required once 'max_supported_bpp' is also a
> u32? Also, what is 18? Shouldn't that be some sort of define so we know
> what it represents?
> 
> Or maybe none of that is required? From what I can tell,
> dp_panel_get_mode_bpp() calls dp_panel_get_supported_bpp() which will
> essentially clamp the bpp range between 18 and 30, unless
> dp_panel->dp_mode.bpp is between 30 and 18 but not divisible by 6, e.g.
> 29. Perhaps this patch can be included and the max_t above dropped.
> 
> ---8<--
> diff --git a/drivers/gpu/drm/msm/dp/dp_panel.c
> b/drivers/gpu/drm/msm/dp/dp_panel.c
> index 07db8f37cd06..5cd7c138afd3 100644
> --- a/drivers/gpu/drm/msm/dp/dp_panel.c
> +++ b/drivers/gpu/drm/msm/dp/dp_panel.c
> @@ -90,22 +90,22 @@ static int dp_panel_read_dpcd(struct dp_panel *dp_panel)
>   static u32 dp_panel_get_supported_bpp(struct dp_panel *dp_panel,
>   		u32 mode_edid_bpp, u32 mode_pclk_khz)
>   {
> -	struct dp_link_info *link_info;
> +	const struct dp_link_info *link_info;
>   	const u32 max_supported_bpp = 30, min_supported_bpp = 18;
> -	u32 bpp = 0, data_rate_khz = 0;
> +	u32 bpp, data_rate_khz;
> 
>   	bpp = min_t(u32, mode_edid_bpp, max_supported_bpp);
> 
>   	link_info = &dp_panel->link_info;
>   	data_rate_khz = link_info->num_lanes * link_info->rate * 8;
> 
> -	while (bpp > min_supported_bpp) {
> +	do {
>   		if (mode_pclk_khz * bpp <= data_rate_khz)
> -			break;
> +			return bpp;
>   		bpp -= 6;
> -	}
> +	} while (bpp > min_supported_bpp);
> 
> -	return bpp;
> +	return min_supported_bpp;
>   }
> 

Thanks for the feedback.

Your change looks valid. We can use this and drop the max_t usage.

Let me push this with your Suggested-by credits.

>   static int dp_panel_update_modes(struct drm_connector *connector,