linux-kernel - [PATCH v3 15/15] media: qcom: camss: Comment CSID dt

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Message-ID: <20230823104444.1954663-16-bryan.odonoghue@linaro.org>
Date:   Wed, 23 Aug 2023 11:44:44 +0100
From:   Bryan O'Donoghue <bryan.odonoghue@...aro.org>
To:     rfoss@...nel.org, todor.too@...il.com, bryan.odonoghue@...aro.org,
        agross@...nel.org, andersson@...nel.org, konrad.dybcio@...aro.org,
        mchehab@...nel.org, hverkuil-cisco@...all.nl,
        laurent.pinchart@...asonboard.com, sakari.ailus@...ux.intel.com,
        andrey.konovalov@...aro.org
Cc:     linux-media@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v3 15/15] media: qcom: camss: Comment CSID dt_id field

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@...aro.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