From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: Bryan O'Donoghue <bryan.odonoghue@linaro.org>,
rfoss@kernel.org, todor.too@gmail.com, agross@kernel.org,
andersson@kernel.org, konrad.dybcio@linaro.org,
mchehab@kernel.org, sakari.ailus@linux.intel.com,
andrey.konovalov@linaro.org, linux-media@vger.kernel.org,
linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 17/17] media: qcom: camss: Comment CSID dt_id field
Date: Mon, 25 Sep 2023 11:14:52 +0300 [thread overview]
Message-ID: <20230925081452.GA8583@pendragon.ideasonboard.com> (raw)
In-Reply-To: <1e815d41-719a-4ca1-98e3-872f882ed03e@xs4all.nl>
On Mon, Sep 25, 2023 at 09:11:52AM +0200, Hans Verkuil wrote:
> On 11/09/2023 15:14, 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
>
> caled -> called
>
> > + * which is a concatenation of [vc:6 | dt_id:2] hence we reuse
>
> vc:6? Do you mean bit 6 or do you mean the least significant 6 bits?
>
> Regardless, since the vc variable <= 3 (since MSM_CSID_MAX_SRC_STREAMS is 4),
> either interpretation makes little sense.
More recent versions of CSI-2 support up to 6 bits of VC (or possibly
even more in versions I may not know about). It would probably make
sense to write vc[5:0] | dt_id[1:0] or something similar.
> And what does "DT" stand for?
Data Type.
> > + * the least significant two bits of the VC to 'stuff' the CID value.
> > + */
> > u8 dt_id = vc;
>
> If dt_id should be the least significant two bits of vc, shouldn't
> this say: "= vc & 3;"? Or, alternatively, have a comment that vc <= 3?
>
> >
> > if (tg->enabled) {
--
Regards,
Laurent Pinchart
next prev parent reply other threads:[~2023-09-25 8:15 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-09-11 13:13 [PATCH v5 00/17] media: qcom: camss: Add parameter passing to remove several outstanding bugs Bryan O'Donoghue
2023-09-11 13:13 ` [PATCH v5 01/17] media: qcom: camss: Amalgamate struct resource with struct resource_ispif Bryan O'Donoghue
2023-09-11 13:13 ` [PATCH v5 02/17] media: qcom: camss: Rename camss struct resources to camss_subdev_resources Bryan O'Donoghue
2023-09-11 13:13 ` [PATCH v5 03/17] media: qcom: camss: Start to move to module compat matched resources Bryan O'Donoghue
2023-09-11 13:13 ` [PATCH v5 04/17] media: qcom: camss: Pass icc bandwidth table as a platform parameter Bryan O'Donoghue
2023-09-11 13:13 ` [PATCH v5 05/17] media: qcom: camss: Pass remainder of variables as resources Bryan O'Donoghue
2023-09-11 13:14 ` [PATCH v5 06/17] media: qcom: camss: Pass line_num from compat resources Bryan O'Donoghue
2023-09-11 13:14 ` [PATCH v5 07/17] media: qcom: camss: Pass CAMSS subdev callbacks via resource ops pointer Bryan O'Donoghue
2023-09-11 13:14 ` [PATCH v5 08/17] media: qcom: camss: Assign the correct number of RDIs per VFE Bryan O'Donoghue
2023-09-11 13:14 ` [PATCH v5 09/17] media: qcom: camss: Remove special case for VFE get/put Bryan O'Donoghue
2023-09-25 8:34 ` Laurent Pinchart
2023-09-11 13:14 ` [PATCH v5 10/17] media: qcom: camss: Untangle if/else spaghetti in camss Bryan O'Donoghue
2023-09-11 13:14 ` [PATCH v5 11/17] media: qcom: camss: Allow clocks vfeN vfe_liteN or vfe_lite Bryan O'Donoghue
2023-09-25 7:10 ` Hans Verkuil
2023-09-25 11:46 ` Bryan O'Donoghue
2023-09-11 13:14 ` [PATCH v5 12/17] media: qcom: camss: Functionally decompose CSIPHY clock lookups Bryan O'Donoghue
2023-09-11 13:14 ` [PATCH v5 13/17] media: qcom: camss: Fix support for setting CSIPHY clock name csiphyX Bryan O'Donoghue
2023-09-11 13:14 ` [PATCH v5 14/17] media: qcom: camss: Support RDI3 for VFE 17x Bryan O'Donoghue
2023-09-11 13:14 ` [PATCH v5 15/17] media: qcom: camss: Move vfe_disable into a common routine where applicable Bryan O'Donoghue
2023-09-11 13:14 ` [PATCH v5 16/17] media: qcom: camss: Propagate vfe_reset error up the callstack Bryan O'Donoghue
2023-09-11 13:14 ` [PATCH v5 17/17] media: qcom: camss: Comment CSID dt_id field Bryan O'Donoghue
2023-09-25 7:11 ` Hans Verkuil
2023-09-25 8:14 ` Laurent Pinchart [this message]
2023-09-25 14:01 ` Bryan O'Donoghue
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