Re: [PATCH v2] media: intel/ipu6: Improve DWC PHY HSFREQRANGE band selection for overlapping ranges

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Hi Marco,

Thanks for the update.

On Wed, Mar 25, 2026 at 10:32:41AM +0100, Marco Nenciarini wrote:
> The get_hsfreq_by_mbps() function searches the freqranges[] table
> backward (from highest to lowest index). Because adjacent frequency
> bands overlap, a data rate that falls in the overlap region always
> lands on the higher-indexed band.
> 
> For data rates up to 1500 Mbps (index 42) every band uses
> osc_freq_target 335. Starting at index 43 (1461-1640 Mbps) the
> osc_freq_target drops to 208. A sensor running at 1498 Mbps sits in
> the overlap between index 42 (1414-1588, osc 335) and index 43
> (1461-1640, osc 208). The backward search picks index 43, programming
> the lower osc_freq_target of 208 instead of the optimal 335.
> 
> This causes DDL lock instability and CSI-2 CRC errors on affected
> configurations, such as the OmniVision OV08X40 sensor on Intel Arrow
> Lake platforms (Dell Pro Max 16).
> 
> Rewrite get_hsfreq_by_mbps() to select the optimal band:
> 
> 1. Prefer an exact default_mbps match (returned immediately).
> 2. Among bands whose min/max range covers the data rate, prefer
>    the one with the higher osc_freq_target.
> 3. If osc_freq_target is equal, prefer the band whose default_mbps
>    is closest to the requested rate.
> 
> For 1498 Mbps this now correctly selects index 42 (osc_freq_target
> 335, range 1414-1588) instead of index 43 (osc_freq_target 208,
> range 1461-1640).
> 
> Fixes: 1e7eeb301696 ("media: intel/ipu6: add the CSI2 DPHY implementation")
> Cc: stable@vger.kernel.org
> Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
> Cc: Bingbu Cao <bingbu.cao@intel.com>
> Signed-off-by: Marco Nenciarini <mnencia@kcore.it>
> ---
> Changes in v2:
> - Rewrote get_hsfreq_by_mbps() with a proper selection algorithm instead
>   of patching after the call, as suggested by Sakari Ailus.
> - Added Fixes tag and Cc stable.
> 
>  .../media/pci/intel/ipu6/ipu6-isys-dwc-phy.c  | 22 ++++++++++++++-----
>  1 file changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/media/pci/intel/ipu6/ipu6-isys-dwc-phy.c b/drivers/media/pci/intel/ipu6/ipu6-isys-dwc-phy.c
> index db28748..4c9e50c 100644
> --- a/drivers/media/pci/intel/ipu6/ipu6-isys-dwc-phy.c
> +++ b/drivers/media/pci/intel/ipu6/ipu6-isys-dwc-phy.c
> @@ -288,15 +288,27 @@ static const struct dwc_dphy_freq_range freqranges[DPHY_FREQ_RANGE_NUM] = {
>  
>  static u16 get_hsfreq_by_mbps(u32 mbps)
>  {
> -	unsigned int i = DPHY_FREQ_RANGE_NUM;
> +	int best = DPHY_FREQ_RANGE_INVALID_INDEX;

The function returns u16 and u16 is fine here, too...

> +	unsigned int i;
>  
> -	while (i--) {
> -		if (freqranges[i].default_mbps == mbps ||
> -		    (mbps >= freqranges[i].min && mbps <= freqranges[i].max))
> +	for (i = 0; i < DPHY_FREQ_RANGE_NUM; i++) {
> +		if (freqranges[i].default_mbps == mbps)
>  			return i;

Doesn't the condition below handle this already?

> +
> +		if (mbps < freqranges[i].min || mbps > freqranges[i].max)
> +			continue;

The frequencies in the array are consistently increasing so you can replace
this with:

		if (mbps < freqranges[i].min)
			continue;

		if (mbps > freqranges[i].max)
			break;

> +
> +		if (best == DPHY_FREQ_RANGE_INVALID_INDEX ||
> +		    freqranges[i].osc_freq_target >
> +		    freqranges[best].osc_freq_target ||
> +		    (freqranges[i].osc_freq_target ==
> +		     freqranges[best].osc_freq_target &&
> +		     abs((int)mbps - (int)freqranges[i].default_mbps) <
> +		     abs((int)mbps - (int)freqranges[best].default_mbps)))
> +			best = i;
>  	}
>  
> -	return DPHY_FREQ_RANGE_INVALID_INDEX;
> +	return best;
>  }
>  
>  static int ipu6_isys_dwc_phy_config(struct ipu6_isys *isys,

-- 
Kind regards,

Sakari Ailus