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

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Date:   Mon, 25 Sep 2023 15:01:44 +0100
From:   Bryan O'Donoghue <bryan.odonoghue@...aro.org>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Hans Verkuil <hverkuil-cisco@...all.nl>
Cc:     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 25/09/2023 09:14, Laurent Pinchart wrote:
> 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) {
> 

I'll rewrite this comment since its still confusing people.

@Hans VC and DT are fields in the CSI protocol structure sections 
2.2.2.2 and 2.2.2.3

https://www.nxp.com/docs/en/application-note/AN5305.pdf

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