| Message ID | 20230823104444.1954663-16-bryan.odonoghue@linaro.org (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | media: qcom: camss: Add parameter passing to remove several outstanding bugs | expand |
On 23.08.2023 12:44, Bryan O'Donoghue wrote: > 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; And where are you discarding the non-2-lsb? Konrad
On 26/08/2023 11:18, Konrad Dybcio wrote: >> + /* >> + * 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; > And where are you discarding the non-2-lsb? At the assignment of dt_id vc:6 dt_id:2 => cid:8 = [vc:6 | dt_id:2] vc == 00110110 cid = [110110 | 10] I have no more information what CID is or how the bitfield is populated than I have already indicated in the comment/commit log. --- bod
On 28.08.2023 17:34, Bryan O'Donoghue wrote: > On 26/08/2023 11:18, Konrad Dybcio wrote: >>> + /* >>> + * 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; >> And where are you discarding the non-2-lsb? > > At the assignment of dt_id > > vc:6 > dt_id:2 > > => > > cid:8 = [vc:6 | dt_id:2] > vc == 00110110 > cid = [110110 | 10] > > I have no more information what CID is or how the bitfield is populated than I have already indicated in the comment/commit log. > OK so you're discarding the 2 lsb of the [vc:6|dt_id:2] however the way I read the comment would suggest that you're taking vc[6:2] Konrad
On 28/08/2023 16:38, Konrad Dybcio wrote: > On 28.08.2023 17:34, Bryan O'Donoghue wrote: >> On 26/08/2023 11:18, Konrad Dybcio wrote: >>>> + /* >>>> + * 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; >>> And where are you discarding the non-2-lsb? >> >> At the assignment of dt_id >> >> vc:6 >> dt_id:2 >> >> => >> >> cid:8 = [vc:6 | dt_id:2] >> vc == 00110110 >> cid = [110110 | 10] >> >> I have no more information what CID is or how the bitfield is populated than I have already indicated in the comment/commit log. >> > OK so you're discarding the 2 lsb of the [vc:6|dt_id:2] > > however > > the way I read the comment would suggest that you're taking > vc[6:2] > > Konrad Fair enough, obvs the comment needs work so. --- 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..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) {
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(+)