| Message ID | 20230925154707.837542-18-bryan.odonoghue@linaro.org |
|---|---|
| State | Superseded |
| Headers | show |
| Series | media: qcom: camss: Add parameter passing to remove several outstanding bugs | expand |
On 27/09/2023 12:42, Hans Verkuil wrote: >> + /* >> + * 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 > This is 5 bits, not 6 bits as the comment above says. Which is right? Yes you're right I had "DT" which is six bits in my head when I wrote this. The VC bitfield is five. > >> + * DT : 21:16 >> + * >> + * CID : VC 3:0 << 2 | DT_ID 1:0 >> + */ >> u8 dt_id = vc; > So if dt_id is 2 bits, and vc can be more than 2 bits in the future, > shouldn't this read "vc & 3"? Hmm... val |= dt_id << RDI_CFG0_DT_ID; yes that would overrun otherwise. --- bod
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) {
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 | 12 ++++++++++++ 1 file changed, 12 insertions(+)