diff mbox series

[v5,17/17] media: qcom: camss: Comment CSID dt_id field

Message ID 20230911131411.196033-18-bryan.odonoghue@linaro.org (mailing list archive)
State Superseded
Headers show
Series media: qcom: camss: Add parameter passing to remove several outstanding bugs | expand

Commit Message

Bryan O'Donoghue Sept. 11, 2023, 1:14 p.m. UTC
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@linaro.org>
---
 drivers/media/platform/qcom/camss/camss-csid-gen2.c | 5 +++++
 1 file changed, 5 insertions(+)

Comments

Hans Verkuil Sept. 25, 2023, 7:11 a.m. UTC | #1
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@linaro.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.

And what does "DT" stand for?

> +		 * 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,

	Hans
Laurent Pinchart Sept. 25, 2023, 8:14 a.m. UTC | #2
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@linaro.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) {
Bryan O'Donoghue Sept. 25, 2023, 2:01 p.m. UTC | #3
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@linaro.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

---
bod
diff mbox series

Patch

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;
 
 		if (tg->enabled) {