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

[hyun.kwon@xilinx.com (Hyun Kwon)]

> -----Original Message-----
> From: dri-devel <dri-devel-bounces@lists.freedesktop.org> On Behalf Of Hyun
> Kwon
> Sent: Thursday, July 30, 2020 7:33 PM
> To: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> Cc: Dan Carpenter <dan.carpenter@oracle.com>; dri-
> devel@lists.freedesktop.org; Daniel Vetter <daniel.vetter@ffwll.ch.ch>
> Subject: Re: [PATCH v2 1/1] drm: xlnx: zynqmp: Use switch - case for link rate
> downshift
> 
> 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.
> 

Applied to drm-misc-next-fixes.

Thanks,
-hyun

> 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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