linux-kernel - Re: [PATCH v3 10/13] media: v4l2-fwnode: Update V4L2_FWNODE_CSI2_MAX_DATA

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Date:   Tue, 8 Dec 2020 21:59:49 +0200
From:   Sakari Ailus <sakari.ailus@....fi>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Sowjanya Komatineni <skomatineni@...dia.com>,
        thierry.reding@...il.com, jonathanh@...dia.com, robh+dt@...nel.org,
        bparrot@...com, mchehab@...nel.org, linux-media@...r.kernel.org,
        linux-tegra@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v3 10/13] media: v4l2-fwnode: Update
 V4L2_FWNODE_CSI2_MAX_DATA_LANES to 8

Hi Hans,

On Mon, Dec 07, 2020 at 11:47:38AM +0100, Hans Verkuil wrote:
> On 03/12/2020 19:59, Sowjanya Komatineni wrote:
> > Some CSI2 receivers support 8 data lanes.
> > 
> > So, this patch updates CSI2 maximum data lanes to be 8.
> > 
> > Signed-off-by: Sowjanya Komatineni <skomatineni@...dia.com>
> > ---
> >  drivers/media/platform/ti-vpe/cal-camerarx.c | 2 +-
> >  include/media/v4l2-fwnode.h                  | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/media/platform/ti-vpe/cal-camerarx.c b/drivers/media/platform/ti-vpe/cal-camerarx.c
> > index 806cbf1..47e2143 100644
> > --- a/drivers/media/platform/ti-vpe/cal-camerarx.c
> > +++ b/drivers/media/platform/ti-vpe/cal-camerarx.c
> > @@ -534,7 +534,7 @@ static int cal_camerarx_parse_dt(struct cal_camerarx *phy)
> >  {
> >  	struct v4l2_fwnode_endpoint *endpoint = &phy->endpoint;
> >  	struct device_node *ep_node;
> > -	char data_lanes[V4L2_FWNODE_CSI2_MAX_DATA_LANES * 2];
> > +	char data_lanes[V4L2_FWNODE_CSI2_MAX_DATA_LANES];
> >  	unsigned int i;
> >  	int ret;
> >  
> 
> I'm not so sure about this change: it relies on the implicit knowledge that
> this cal driver can handle only 4 lanes max, so that doubling
> V4L2_FWNODE_CSI2_MAX_DATA_LANES is the same as the old 'V4L2_FWNODE_CSI2_MAX_DATA_LANES * 2'.
> 
> I think we should either keep the existing code (which means data_lanes
> is now larger than needed, so stack usage increases by 8 bytes), or perhaps
> create a new define for this driver like CAL_MAX_DATA_LANES and use that.
> 
> In my opinion the original code should just be kept, but I've CC-ed Laurent
> on this to hear what he thinks.

I further looked at the code there and it does *depend* on a particular
value of V4L2_FWNODE_CSI2_MAX_DATA_LANES. That needs to be fixed. This part
can (or should) be dropped from the patch though.

-- 
Regards,

Sakari Ailus