Re: [PATCH v3 15/15] media: qcom: camss: Comment CSID dt_id field

Konrad Dybcio
2023-08-26T10:19:31Z

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@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
> + * 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