[alsa-devel] [PATCH] Sound: sgtl5000 Allow codec clock frequency to be set.

Timur Tabi timur at tabi.org
Thu Mar 21 18:52:19 CET 2013

On 03/21/2013 12:46 PM, Martin Fuzzey wrote:
> On 21/03/13 17:11, Timur Tabi wrote:
>> On Thu, Mar 21, 2013 at 3:39 AM, Martin Fuzzey <mfuzzey at parkeon.com> wrote:
>>> With this patch and that DT example the frequency of clock 162 will be _set_
>>> to 20MHz
>> Does that mean that it ignores the ' clocks' parameter?
> No, in this case the clocks parameter must contain the phandle of a clock
> whose rate is configurable.
>> If clock-frequency is omitted the binding is still correct (hence the
>> optional) but the frequency of clock 162 would not be modified.
>> In the documentation I wrote "If both a clock and clock-frequency are
>> provided the clock's rate will be set. " maybe this is not clear enough?
>> It's not clear what it will be set *to*.
> It will be set to the frequency specified by the clock-frquency attribute.
> Would
> "If both a clock and clock-frequency are provided clock must support
> the set_rate operation and its frequency will be set to the value specified
> by clock-frequency" be better?

Yes, that's much better.

> The possible configurations and their use cases are:
> 1) only 'clocks' specified
> clock points to a clock specified in the DT which already has an 
> appropriate frequency and is
> configured by some other means external to the sgtl5000 driver 
> (bootloader, board setup code, or just a fixed rate clock)
> 2) only 'clock-frequency' specified
> The chip is assumed to be clocked by a signal having the given 
> frequency, which may even be generated by a clock unknown to linux.
> This could actually be represented as a special case of 1) by defining a 
> fixed-rate clock in the DT.
> 3) Both 'clocks' and 'clock-frequency' specified
> The chip is assumed to be clocked by a rate programmable clock defined 
> in the clock tree.
> clk_set_rate() will be called for this clock to set its rate to that 
> specified by clock-frequency

This should all be in the binding document.

> Cases 1 and 2 exist in the current code, this patch adds support for case 3.
> Prior to commit 81e8e4926167ab32593bbb915b45a42024ca1020 "ASoC: fsl: add 
> sgtl5000 clock support for imx-sgtl5000"
> only case 2 was supported
> This explains why 2 is not implemented as a special case of 1)
> plain 'clock-frequency' was supported before the driver learnt how to 
> get a clock from the DT

And this should be in the patch description.

Timur Tabi

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