Re: [PATCH v2 1/1] drm: xlnx: zynqmp: Use switch - case for link rate downshift

[Hyun Kwon <hyun.kwon@xilinx.com>]

Hi Laurent,

Thanks for the comment.

On Thu, Jul 30, 2020 at 04:12:46PM -0700, Laurent Pinchart wrote:
> Hi Hyun,
> 
> Thank you for the patch.
> 
> On Wed, Jul 29, 2020 at 04:30:45PM -0700, Hyun Kwon wrote:
> > Use switch - case to downshift from the current link rate. It's a small
> > loop now, so fine to be replaced with switch - case. With a loop, it is
> > confusing and hard to follow as reported below.
> > 
> > The patch d76271d22694: "drm: xlnx: DRM/KMS driver for Xilinx ZynqMP
> > DisplayPort Subsystem" from Jul 7, 2018, leads to the following
> > static checker warning:
> > 
> > 	drivers/gpu/drm/xlnx/zynqmp_dp.c:594 zynqmp_dp_mode_configure()
> > 	error: iterator underflow 'bws' (-1)-2
> > 
> > Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> > Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
> > ---
> > v2
> > - Convert the for loop into switch - case
> > ---
> > ---
> >  drivers/gpu/drm/xlnx/zynqmp_dp.c | 29 ++++++++++++++++-------------
> >  1 file changed, 16 insertions(+), 13 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/xlnx/zynqmp_dp.c b/drivers/gpu/drm/xlnx/zynqmp_dp.c
> > index b735072..5d6adeaa 100644
> > --- a/drivers/gpu/drm/xlnx/zynqmp_dp.c
> > +++ b/drivers/gpu/drm/xlnx/zynqmp_dp.c
> > @@ -567,34 +567,37 @@ static int zynqmp_dp_mode_configure(struct zynqmp_dp *dp, int pclock,
> >  				    u8 current_bw)
> >  {
> >  	int max_rate = dp->link_config.max_rate;
> > -	u8 bws[3] = { DP_LINK_BW_1_62, DP_LINK_BW_2_7, DP_LINK_BW_5_4 };
> > +	u8 bw_code;
> >  	u8 max_lanes = dp->link_config.max_lanes;
> >  	u8 max_link_rate_code = drm_dp_link_rate_to_bw_code(max_rate);
> >  	u8 bpp = dp->config.bpp;
> >  	u8 lane_cnt;
> > -	s8 i;
> >  
> > -	if (current_bw == DP_LINK_BW_1_62) {
> > +	/* Downshift from current one */
> 
> Maybe "Downshift from the current bandwidth" ?
> 
> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> 

I agree. I'll fix and add the tag.

Thanks!

-hyun

> > +	switch (current_bw) {
> > +	case DP_LINK_BW_5_4:
> > +		bw_code = DP_LINK_BW_2_7;
> > +		break;
> > +	case DP_LINK_BW_2_7:
> > +		bw_code = DP_LINK_BW_1_62;
> > +		break;
> > +	case DP_LINK_BW_1_62:
> >  		dev_err(dp->dev, "can't downshift. already lowest link rate\n");
> >  		return -EINVAL;
> > -	}
> > -
> > -	for (i = ARRAY_SIZE(bws) - 1; i >= 0; i--) {
> > -		if (current_bw && bws[i] >= current_bw)
> > -			continue;
> > -
> > -		if (bws[i] <= max_link_rate_code)
> > -			break;
> > +	default:
> > +		/* If not given, start with max supported */
> > +		bw_code = max_link_rate_code;
> > +		break;
> >  	}
> >  
> >  	for (lane_cnt = 1; lane_cnt <= max_lanes; lane_cnt <<= 1) {
> >  		int bw;
> >  		u32 rate;
> >  
> > -		bw = drm_dp_bw_code_to_link_rate(bws[i]);
> > +		bw = drm_dp_bw_code_to_link_rate(bw_code);
> >  		rate = zynqmp_dp_max_rate(bw, lane_cnt, bpp);
> >  		if (pclock <= rate) {
> > -			dp->mode.bw_code = bws[i];
> > +			dp->mode.bw_code = bw_code;
> >  			dp->mode.lane_cnt = lane_cnt;
> >  			dp->mode.pclock = pclock;
> >  			return dp->mode.bw_code;
> 
> -- 
> Regards,
> 
> Laurent Pinchart
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