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media: qcom: camss: Comment CSID dt_id field

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commit f910d3ba78 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 3c4ed72a16 ("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>
This commit is contained in:
Bryan O'Donoghue 2023-09-28 01:58:25 +01:00 committed by Greg Kroah-Hartman
parent c897fb3da8
commit e53b78e3cf
1 changed files with 13 additions and 1 deletions
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14
drivers/media/platform/qcom/camss/camss-csid-gen2.c
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@ -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 */