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[188.141.3.169]) by smtp.gmail.com with ESMTPSA id d15-20020adffd8f000000b00317ab75748bsm23822145wrr.49.2023.09.07.09.44.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 07 Sep 2023 09:44:36 -0700 (PDT) From: Bryan O'Donoghue To: rfoss@kernel.org, todor.too@gmail.com, bryan.odonoghue@linaro.org, agross@kernel.org, andersson@kernel.org, konrad.dybcio@linaro.org, mchehab@kernel.org, hverkuil-cisco@xs4all.nl, laurent.pinchart@ideasonboard.com, sakari.ailus@linux.intel.com, andrey.konovalov@linaro.org Cc: linux-media@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v4 17/17] media: qcom: camss: Comment CSID dt_id field Date: Thu, 7 Sep 2023 17:44:10 +0100 Message-ID: <20230907164410.36651-18-bryan.odonoghue@linaro.org> X-Mailer: git-send-email 2.42.0 In-Reply-To: <20230907164410.36651-1-bryan.odonoghue@linaro.org> References: <20230907164410.36651-1-bryan.odonoghue@linaro.org> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-media@vger.kernel.org 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 --- 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; if (tg->enabled) {