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| author | Sakari Ailus <sakari.ailus@linux.intel.com> | 2020-12-11 20:56:04 +0300 |
|---|---|---|
| committer | Mauro Carvalho Chehab <mchehab+huawei@kernel.org> | 2021-01-12 19:09:02 +0300 |
| commit | 2225cf449294ebba4e03a300636e1a8b20953c0b (patch) | |
| tree | 85f4a991475be7a9f1c75d5d6ba7d0f4a9b30eaa /Documentation/driver-api/media | |
| parent | 3ef5e42d281ea108f4ccdca186de4ce20a346326 (diff) | |
| download | linux-2225cf449294ebba4e03a300636e1a8b20953c0b.tar.xz | |
media: Documentation: media: Document clock handling in camera sensor drivers
Document pratices of handling clocks in camera sensor drivers on both DT
and ACPI.
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Diffstat (limited to 'Documentation/driver-api/media')
| -rw-r--r-- | Documentation/driver-api/media/camera-sensor.rst | 20 |
1 files changed, 19 insertions, 1 deletions
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 ---------- |