| Message ID | 20230925154707.837542-18-bryan.odonoghue@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | media: qcom: camss: Add parameter passing to remove several outstanding bugs | expand |
On 25/09/2023 17:47, 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 | 12 ++++++++++++ > 1 file changed, 12 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..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 This is 5 bits, not 6 bits as the comment above says. Which is right? > + * 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"? Regards, Hans > > if (tg->enabled) {
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
On 27/09/2023 14:03, Bryan O'Donoghue wrote: > 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. No need to repost the whole series, just post a v6.1 for this patch only. Regards, Hans
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(+)