linux-kernel - Re: [RESEND PATCH v6 17/17] media: qcom: camss: Comment CSID dt

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Date:   Wed, 27 Sep 2023 13:42:24 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Bryan O'Donoghue <bryan.odonoghue@...aro.org>,
        linux-arm-msm@...r.kernel.org, linux-media@...r.kernel.org,
        laurent.pinchart@...asonboard.com, rfoss@...nel.org,
        todor.too@...il.com, agross@...nel.org, andersson@...nel.org,
        konrad.dybcio@...aro.org, mchehab@...nel.org,
        sakari.ailus@...ux.intel.com, andrey.konovalov@...aro.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v6 17/17] media: qcom: camss: Comment CSID dt_id
 field

On 25/09/2023 17:47, Bryan O'Donoghue wrote:
> Digging into the documentation we find that the DT_ID bitfield is used to
> map the six bit DT to a two bit ID code. This value is concatenated to the
> VC bitfield to create a CID value. DT_ID is the two least significant bits
> of CID and VC the most significant bits.
> 
> Originally we set dt_id = vc * 4 in and then subsequently set dt_id = vc.
> 
> commit 3c4ed72a16bc ("media: camss: sm8250: Virtual channels for CSID")
> silently fixed the multiplication by four which would give a better
> value for the generated CID without mentioning what was being done or why.
> 
> Next up I haplessly changed the value back to "dt_id = vc * 4" since there
> didn't appear to be any logic behind it.
> 
> Hans asked what the change was for and I honestly couldn't remember the
> provenance of it, so I dug in.
> 
> Link: https://lore.kernel.org/linux-arm-msm/edd4bf9b-0e1b-883c-1a4d-50f4102c3924@xs4all.nl/
> 
> Add a comment so the next hapless programmer doesn't make this same
> mistake.
> 
> Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@...aro.org>
> ---
>  drivers/media/platform/qcom/camss/camss-csid-gen2.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/media/platform/qcom/camss/camss-csid-gen2.c b/drivers/media/platform/qcom/camss/camss-csid-gen2.c
> index 6ba2b10326444..791f27b18c394 100644
> --- a/drivers/media/platform/qcom/camss/camss-csid-gen2.c
> +++ b/drivers/media/platform/qcom/camss/camss-csid-gen2.c
> @@ -352,6 +352,18 @@ static void __csid_configure_stream(struct csid_device *csid, u8 enable, u8 vc)
>  		phy_sel = csid->phy.csiphy_id;
>  
>  	if (enable) {
> +		/*
> +		 * DT_ID is a two bit bitfield that is concatenated with
> +		 * the four least significant bits of the six bit VC
> +		 * bitfield to generate an internal CID value.
> +		 *
> +		 * CSID_RDI_CFG0(vc)
> +		 * DT_ID : 28:27
> +		 * VC    : 26:22

This is 5 bits, not 6 bits as the comment above says. Which is right?

> +		 * DT    : 21:16
> +		 *
> +		 * CID   : VC 3:0 << 2 | DT_ID 1:0
> +		 */
>  		u8 dt_id = vc;

So if dt_id is 2 bits, and vc can be more than 2 bits in the future,
shouldn't this read "vc & 3"?

Regards,

	Hans

>  
>  		if (tg->enabled) {