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
+ * DT : 21:16
+ *
+ * CID : VC 3:0 << 2 | DT_ID 1:0
+ */
u8 dt_id = vc;
if (tg->enabled) {
--
2.42.0
