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    Subject[RESEND PATCH v6 17/17] media: qcom: camss: Comment CSID dt_id field
    Date
    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 | 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
    + * DT : 21:16
    + *
    + * CID : VC 3:0 << 2 | DT_ID 1:0
    + */
    u8 dt_id = vc;

    if (tg->enabled) {
    --
    2.42.0
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