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[188.141.3.169]) by smtp.gmail.com with ESMTPSA id v21-20020a7bcb55000000b00404719b05b5sm18789086wmj.27.2023.09.27.17.58.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 27 Sep 2023 17:58:44 -0700 (PDT) From: Bryan O'Donoghue To: linux-arm-msm@vger.kernel.org, linux-media@vger.kernel.org, hverkuil@xs4all.nl, laurent.pinchart@ideasonboard.com, 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 Cc: linux-kernel@vger.kernel.org, Bryan O'Donoghue Subject: [PATCH v6.1] media: qcom: camss: Comment CSID dt_id field Date: Thu, 28 Sep 2023 01:58:25 +0100 Message-ID: <20230928005825.1626251-1-bryan.odonoghue@linaro.org> X-Mailer: git-send-email 2.42.0 In-Reply-To: <314cee42-c235-4d2a-b4a6-4da92d2c1548@xs4all.nl> References: <314cee42-c235-4d2a-b4a6-4da92d2c1548@xs4all.nl> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-arm-msm@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 --- .../media/platform/qcom/camss/camss-csid-gen2.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) diff --git a/drivers/media/platform/qcom/camss/camss-csid-gen2.c b/drivers/media/platform/qcom/camss/camss-csid-gen2.c index 6ba2b10326444..05ff5fa8095a8 100644 --- a/drivers/media/platform/qcom/camss/camss-csid-gen2.c +++ b/drivers/media/platform/qcom/camss/camss-csid-gen2.c @@ -352,7 +352,19 @@ static void __csid_configure_stream(struct csid_device *csid, u8 enable, u8 vc) phy_sel = csid->phy.csiphy_id; if (enable) { - u8 dt_id = vc; + /* + * DT_ID is a two bit bitfield that is concatenated with + * the four least significant bits of the five 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 & 0x03; if (tg->enabled) { /* configure one DT, infinite frames */