linux-kernel - Re: [PATCH v5 17/17] media: qcom: camss: Comment CSID dt

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Message-ID: <20230925081452.GA8583@pendragon.ideasonboard.com>
Date:   Mon, 25 Sep 2023 11:14:52 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Hans Verkuil <hverkuil-cisco@...all.nl>
Cc:     Bryan O'Donoghue <bryan.odonoghue@...aro.org>, 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,
        linux-media@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 17/17] media: qcom: camss: Comment CSID dt_id field

On Mon, Sep 25, 2023 at 09:11:52AM +0200, Hans Verkuil wrote:
> On 11/09/2023 15:14, 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
> 
> caled -> called
> 
> > +		 * which is a concatenation of [vc:6 | dt_id:2] hence we reuse
> 
> vc:6? Do you mean bit 6 or do you mean the least significant 6 bits?
> 
> Regardless, since the vc variable <= 3 (since MSM_CSID_MAX_SRC_STREAMS is 4),
> either interpretation makes little sense.

More recent versions of CSI-2 support up to 6 bits of VC (or possibly
even more in versions I may not know about). It would probably make
sense to write vc[5:0] | dt_id[1:0] or something similar.

> And what does "DT" stand for?

Data Type.

> > +		 * the least significant two bits of the VC to 'stuff' the CID value.
> > +		 */
> >  		u8 dt_id = vc;
> 
> If dt_id should be the least significant two bits of vc, shouldn't
> this say: "= vc & 3;"? Or, alternatively, have a comment that vc <= 3?
> 
> >  
> >  		if (tg->enabled) {

-- 
Regards,

Laurent Pinchart