On 25/09/2023 17:47, 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@xxxxxxxxx/
>
> Add a comment so the next hapless programmer doesn't make this same
> mistake.
>
> Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@xxxxxxxxxx>
> ---
> drivers/media/platform/qcom/camss/camss-csid-gen2.c | 12 ++++++++++++
> 1 file changed, 12 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..791f27b18c394 100644
> --- a/drivers/media/platform/qcom/camss/camss-csid-gen2.c
> +++ b/drivers/media/platform/qcom/camss/camss-csid-gen2.c
> @@ -352,6 +352,18 @@ static void __csid_configure_stream(struct csid_device *csid, u8 enable, u8 vc)
> phy_sel = csid->phy.csiphy_id;
>
> if (enable) {
> + /*
> + * DT_ID is a two bit bitfield that is concatenated with
> + * the four least significant bits of the six bit VC
> + * bitfield to generate an internal CID value.
> + *
> + * CSID_RDI_CFG0(vc)
> + * DT_ID : 28:27
> + * VC : 26:22
This is 5 bits, not 6 bits as the comment above says. Which is right?
> + * DT : 21:16
> + *
> + * CID : VC 3:0 << 2 | DT_ID 1:0
> + */
> u8 dt_id = vc;
So if dt_id is 2 bits, and vc can be more than 2 bits in the future,
shouldn't this read "vc & 3"?
Regards,
Hans
>
> if (tg->enabled) {
