From: Sakari Ailus <sakari.ailus@linux.intel.com>
To: linux-media@vger.kernel.org
Cc: devicetree@vger.kernel.org, linux-acpi@vger.kernel.org,
linux-clk@vger.kernel.org
Subject: [PATCH 1/1] Documentation: media: Document clock handling in camera sensor drivers
Date: Mon, 21 Dec 2020 17:25:29 +0200 [thread overview]
Message-ID: <20201221152529.25417-1-sakari.ailus@linux.intel.com> (raw)
Document pratices of handling clocks in camera sensor drivers on both DT
and ACPI.
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
---
.../driver-api/media/camera-sensor.rst | 20 ++++++++++++++++++-
1 file changed, 19 insertions(+), 1 deletion(-)
diff --git a/Documentation/driver-api/media/camera-sensor.rst b/Documentation/driver-api/media/camera-sensor.rst
index ffb0cad8137a..4e8e92fdd8b4 100644
--- a/Documentation/driver-api/media/camera-sensor.rst
+++ b/Documentation/driver-api/media/camera-sensor.rst
@@ -15,7 +15,7 @@ Camera sensors have an internal clock tree including a PLL and a number of
divisors. The clock tree is generally configured by the driver based on a few
input parameters that are specific to the hardware:: the external clock frequency
and the link frequency. The two parameters generally are obtained from system
-firmware. No other frequencies should be used in any circumstances.
+firmware. **No other frequencies should be used in any circumstances.**
The reason why the clock frequencies are so important is that the clock signals
come out of the SoC, and in many cases a specific frequency is designed to be
@@ -23,6 +23,24 @@ used in the system. Using another frequency may cause harmful effects
elsewhere. Therefore only the pre-determined frequencies are configurable by the
user.
+ACPI
+~~~~
+
+Read the "clock-frequency" _DSD property to denote the frequency. The driver can
+rely this frequency being used.
+
+Devicetree
+~~~~~~~~~~
+
+The currently preferred way to achieve this is using "assigned-clock-rates"
+property. See Documentation/devicetree/bindings/clock/clock-bindings.txt for
+more information.
+
+This approach has the drawback that there's no guarantee that the frequency
+hasn't been modified directly or indirectly by another driver, or supported by
+the board's clock tree to begin with. Changes to the Common Clock Framework API
+are required to ensure reliability.
+
Frame size
----------
--
2.29.2
reply other threads:[~2020-12-21 15:33 UTC|newest]
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