[PATCH 5/6] clk: bcm2835: correctly enable fractional clock support

Eric Anholt eric at anholt.net
Mon Feb 29 15:44:16 PST 2016

Martin Sperl <kernel at martin.sperl.org> writes:

>> On 29.02.2016, at 21:33, Eric Anholt <eric at anholt.net> wrote:
>> kernel at martin.sperl.org writes:
>>> From: Martin Sperl <kernel at martin.sperl.org>
>>> The current driver calculates the clock divider with
>>> fractional support enabled.
>>> But it does not enable fractional support in the
>>> control register itself resulting in an integer only divider,
>>> but in clk_set_rate responds back the fractionally divided
>>> clock frequency.
>>> This patch enables fractional support in the control register
>>> whenever there is a fractional bit set in the requested clock divider.
>>> Mash clock limits are are also handled for the PWM clock
>>> applying the correct divider limits (2 and max_int) applicable to
>>> basic fractional divider support (mash order of 1).
>>> It also adds locking to protect the read/modify/write cycle of
>>> the register modification.
>>> Fixes: 41691b8862e2 ("clk: bcm2835: Add support for programming the
>>> audio domain clocks")
>>> Signed-off-by: Martin Sperl <kernel at martin.sperl.org>
>> Rewrite of the commit message:
>> The driver has been using fractional dividers for all clocks with
>> fractional bits.  However, MASH clocks (those used to drive audio) only
>> do fractional division when in MASH mode, which is used to push clock
>> jitter out of the audio band.  The PWM clock (used to drive i2s audio)
>> is the only MASH clock currently enabled in the driver.
>> Additional MASH modes are available that widen the frequency range, but
>> only 1-level MASH is used for now, until we characterize some better
>> policy.
>> Fixes: 41691b8862e2 ("clk: bcm2835: Add support for programming the
>> audio domain clocks”)
> That is not 100% true - non mash modes have CM_FRAC and CM_FRAC = CM_MASH0
> so these map each other

Once again, trusting the docs turns out to be a bad idea.  You're right,
the non-MASH clocks *do* have a bit 9 to enable fractional mode.  Sigh.

So, this patch is:

Reviewed-by: Eric Anholt <eric at anholt.net>
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