linux-kernel - Re: [PATCH v3 15/15] media: qcom: camss: Comment CSID dt

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Message-ID: <4929aa72-a134-4eeb-850e-46d9255c011b@linaro.org>
Date:   Sat, 26 Aug 2023 12:18:26 +0200
From:   Konrad Dybcio <konrad.dybcio@...aro.org>
To:     Bryan O'Donoghue <bryan.odonoghue@...aro.org>, rfoss@...nel.org,
        todor.too@...il.com, agross@...nel.org, andersson@...nel.org,
        mchehab@...nel.org, hverkuil-cisco@...all.nl,
        laurent.pinchart@...asonboard.com, sakari.ailus@...ux.intel.com,
        andrey.konovalov@...aro.org
Cc:     linux-media@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 15/15] media: qcom: camss: Comment CSID dt_id field

On 23.08.2023 12:44, 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 | 5 +++++
>  1 file changed, 5 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..cee50fc87e9de 100644
> --- a/drivers/media/platform/qcom/camss/camss-csid-gen2.c
> +++ b/drivers/media/platform/qcom/camss/camss-csid-gen2.c
> @@ -352,6 +352,11 @@ static void __csid_configure_stream(struct csid_device *csid, u8 enable, u8 vc)
>  		phy_sel = csid->phy.csiphy_id;
>  
>  	if (enable) {
> +		/*
> +		 * A value caled 'CID' gets generated internal to CAMSS logic
> +		 * which is a concatenation of [vc:6 | dt_id:2] hence we reuse
> +		 * the least significant two bits of the VC to 'stuff' the CID value.
> +		 */
>  		u8 dt_id = vc;
And where are you discarding the non-2-lsb?

Konrad