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[RESEND,v6,17/17] media: qcom: camss: Comment CSID dt_id field

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State Superseded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

Commit Message

Bryan O'Donoghue Sept. 25, 2023, 3:47 p.m. UTC
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(+)

Comments

Bryan O'Donoghue Sept. 27, 2023, 12:03 p.m. UTC | #1
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
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Patch

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) {