media: qcom: camss: Comment CSID dt_id field

commit f910d3b upstream.

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 3c4ed72 ("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>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/media/platform/qcom/camss/camss-csid-gen2.c

@@ -352,7 +352,19 @@ static void __csid_configure_stream(struct csid_device *csid, u8 enable, u8 vc)
 		phy_sel = csid->phy.csiphy_id;
 
 	if (enable) {
-		u8 dt_id = vc;
+		/*
+		 * DT_ID is a two bit bitfield that is concatenated with
+		 * the four least significant bits of the five 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 & 0x03;
 
 		if (tg->enabled) {
 			/* configure one DT, infinite frames */