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

Bryan O'Donoghue
2023-08-23T10:46:02Z

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;

if (tg->enabled) {
--
2.41.0