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

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Date:   Mon, 7 Dec 2020 11:47:38 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Sowjanya Komatineni <skomatineni@...dia.com>,
        thierry.reding@...il.com, jonathanh@...dia.com,
        sakari.ailus@....fi, robh+dt@...nel.org
Cc:     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

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.

Regards,

	Hans

> diff --git a/include/media/v4l2-fwnode.h b/include/media/v4l2-fwnode.h
> index 4e1f6e1d..92401c1 100644
> --- a/include/media/v4l2-fwnode.h
> +++ b/include/media/v4l2-fwnode.h
> @@ -25,7 +25,7 @@ struct fwnode_handle;
>  struct v4l2_async_notifier;
>  struct v4l2_async_subdev;
>  
> -#define V4L2_FWNODE_CSI2_MAX_DATA_LANES	4
> +#define V4L2_FWNODE_CSI2_MAX_DATA_LANES	8
>  
>  /**
>   * struct v4l2_fwnode_bus_mipi_csi2 - MIPI CSI-2 bus data structure
>