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Date:   Wed, 10 Jun 2020 15:40:04 +0200
From:   Niklas =?iso-8859-1?Q?S=F6derlund?= 
        <niklas.soderlund@ragnatech.se>
To:     Suresh Udipi <sudipi@jp.adit-jv.com>
Cc:     mrodin@de.adit-jv.com, efriedrich@de.adit-jv.com,
        erosca@de.adit-jv.com, linux-kernel@vger.kernel.org,
        linux-media@vger.kernel.org, linux-renesas-soc@vger.kernel.org,
        mchehab@kernel.org, michael@rodin.online,
        securitycheck@denso.co.jp,
        Kazuyoshi Akiyama <akiyama@nds-osk.co.jp>
Subject: Re: [PATCH v6] media: rcar-csi2: Correct the selection of hsfreqrange
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Hi Suresh,

Thanks for your persistent work!

On 2020-06-08 12:25:03 +0900, Suresh Udipi wrote:
> hsfreqrange should be chosen based on the calculated mbps which
> is closer to the default bit rate  and within the range as per
> table[1]. But current calculation always selects first value which
> is greater than or equal to the calculated mbps which may lead
> to chosing a wrong range in some cases.
> 
> For example for 360 mbps for H3/M3N
> Existing logic selects
> Calculated value 360Mbps : Default 400Mbps Range [368.125 -433.125 mbps]
> 
> This hsfreqrange is out of range.
> 
> The logic is changed to get the default value which is closest to the
> calculated value [1]
> 
> Calculated value 360Mbps : Default 350Mbps  Range [320.625 -380.625 mpbs]
> 
> [1] specs r19uh0105ej0200-r-car-3rd-generation.pdf [Table 25.9]
> 
> Please note that According to Renesas in Table 25.9 the range for
> 220 default value is corrected as below
> 
>  |Range (Mbps)     |  Default  Bit rate (Mbps) |
>  -----------------------------------------------
>  | 197.125-244.125 |     220                   |
>  -----------------------------------------------
> 
> Fixes: 769afd212b16 ("media: rcar-csi2: add Renesas R-Car MIPI CSI-2 receiver driver")
> 
> Signed-off-by: Suresh Udipi <sudipi@jp.adit-jv.com>
> Signed-off-by: Kazuyoshi Akiyama <akiyama@nds-osk.co.jp>
> Signed-off-by: Michael Rodin <mrodin@de.adit-jv.com>
> ---
>  Changes in v2:
>   - Added the boundary check for the maximum bit rate.
> 
>   - Simplified the logic by remmoving range check
>     as only the closest default value covers most
>     of the use cases.
> 
>   - Aligning the commit message based on the above change
> 
> 
>  Changes in v3:
>     - Added max member from struct rcsi2_mbps_reg.
>       mbps varialbe cannot be removed from rcsi2_mbps_reg,
>       since this structure is reused for
>       phtw_mbps_h3_v3h_m3n/phtw_mbps_v3m_e3 where mbps is
>       used.
> 
> 
>    -  Update the walk of the array in rcsi2_set_phypll() so that it finds
>       the first entry where the calculated bit rate is less than the max.
> 
>    - Support lower bit rates less than 80Mbps like 48Mbps
>      (Raspberry pi camera 640x480 connected to Kingfisher)
>      can also be supported by selecting the lowest default bit rate 80Mbps
>      as done before this fix
> 
>    - Alignement of the commit message based on above changes.
> 
>  Changes in v4:
>   -  Remove unncessary braces.
> 
>  Changes in v5:
>    - Removed mbps variable in rcsi2_mbps_reg and aligned all 
>      tables accordingly
> 	 
>  Changes in v6
>    - Renesas correct the range of default value 220Mbps. Now
>      if we select the nearest value to the default value all
> 	 the values are in range. So reverting back to original patch
> 	 
>    - Added warning for values less than Minimum 80Mbps
> 
> 
>  drivers/media/platform/rcar-vin/rcar-csi2.c | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/media/platform/rcar-vin/rcar-csi2.c b/drivers/media/platform/rcar-vin/rcar-csi2.c
> index 151e6a9..8c502b7 100644
> --- a/drivers/media/platform/rcar-vin/rcar-csi2.c
> +++ b/drivers/media/platform/rcar-vin/rcar-csi2.c
> @@ -199,6 +199,8 @@ static const struct rcsi2_mbps_reg phtw_mbps_v3m_e3[] = {
>  /* PHY Frequency Control */
>  #define PHYPLL_REG			0x68
>  #define PHYPLL_HSFREQRANGE(n)		((n) << 16)
> +#define PHYPLL_HSFREQRANGE_MAX		1500
> +#define PHYPLL_HSFREQRANGE_MIN		  80
>  
>  static const struct rcsi2_mbps_reg hsfreqrange_h3_v3h_m3n[] = {
>  	{ .mbps =   80, .reg = 0x00 },
> @@ -431,16 +433,27 @@ static int rcsi2_wait_phy_start(struct rcar_csi2 *priv)
>  static int rcsi2_set_phypll(struct rcar_csi2 *priv, unsigned int mbps)
>  {
>  	const struct rcsi2_mbps_reg *hsfreq;
> +	const struct rcsi2_mbps_reg *hsfreq_prev = NULL;
>  
> -	for (hsfreq = priv->info->hsfreqrange; hsfreq->mbps != 0; hsfreq++)
> -		if (hsfreq->mbps >= mbps)
> -			break;
> -
> -	if (!hsfreq->mbps) {
> +	if (mbps > PHYPLL_HSFREQRANGE_MAX) {
>  		dev_err(priv->dev, "Unsupported PHY speed (%u Mbps)", mbps);
>  		return -ERANGE;
>  	}
>  
> +	if (mbps < PHYPLL_HSFREQRANGE_MIN)
> +		dev_warn(priv->dev, "PHY speed (%u Mbps) less \
> +			 than Min 80Mbps\n", mbps);

I would drop this warning.

> +
> +	for (hsfreq = priv->info->hsfreqrange; hsfreq->mbps != 0; hsfreq++) {
> +		if (hsfreq->mbps >= mbps)
> +			break;
> +		hsfreq_prev = hsfreq;
> +	}
> +
> +	if (hsfreq_prev &&
> +	    ((mbps - hsfreq_prev->mbps) <= (hsfreq->mbps - mbps)))

Longer lines are now OK [1] and I think it would add to the readability 
here.

> +		hsfreq = hsfreq_prev;
> +

How about

static int rcsi2_set_phypll(struct rcar_csi2 *priv, unsigned int mbps)
{
    const struct rcsi2_mbps_reg *hsfreq;
    const struct rcsi2_mbps_reg *hsfreq_prev = NULL;

    for (hsfreq = priv->info->hsfreqrange; hsfreq->mbps != 0; hsfreq++) {
	if (hsfreq->mbps >= mbps)
		break;
	hsfreq_prev = hsfreq;
    }

    if (!hsfreq->mbps) {
	dev_err(priv->dev, "Unsupported PHY speed (%u Mbps)", mbps);
	return -ERANGE;
    }

    if (hsfreq_prev && ((mbps - hsfreq_prev->mbps) <= (hsfreq->mbps - mbps)))
	hsfreq = hsfreq_prev;

    rcsi2_write(priv, PHYPLL_REG, PHYPLL_HSFREQRANGE(hsfreq->reg));

    return 0;
}

>  	rcsi2_write(priv, PHYPLL_REG, PHYPLL_HSFREQRANGE(hsfreq->reg));
>  
>  	return 0;
> -- 
> 2.7.4
> 

1. https://lkml.org/lkml/2020/5/29/1038

-- 
Regards,
Niklas S?derlund