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Subject: media: Documentation: media: Document clock handling in camera sensor drivers Author: Sakari Ailus <[email protected]> Date: Fri Dec 11 18:56:04 2020 +0100 Document pratices of handling clocks in camera sensor drivers on both DT and ACPI. Signed-off-by: Sakari Ailus <[email protected]> Signed-off-by: Mauro Carvalho Chehab <[email protected]> Documentation/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..3fc378b3b269 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 on 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. The driver then gets the frequency using clk_get_rate(). + +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 ---------- _______________________________________________ linuxtv-commits mailing list [email protected] https://www.linuxtv.org/cgi-bin/mailman/listinfo/linuxtv-commits
