[PATCH v2] clk-divider: make sure read-only dividers do not write to their register

[sboyd@codeaurora.org (Stephen Boyd)]

On 01/20, Heiko Stuebner wrote:
> Commit e6d5e7d90be9 ("clk-divider: Fix READ_ONLY when divider > 1") removed
> the special ops struct for read-only clocks and instead opted to handle
> them regularly.
> 
> On the rk3368 this results in breakage as aclkm now gets set a value.
> While it is the same divider value, the A53 core still doesn't like it,
> which can result in the cpu ending up in a hang.
> The reason being that "ACLKENMasserts one clock cycle before the rising
> edge of ACLKM" and the clock should only be touched when STANDBYWFIL2
> is asserted.
> 
> So make sure read-only clocks don't touch the clock-register at all
> even if only writing the same value, as even writing the same value
> may not be safe in all cases.
> 
> Fixes: e6d5e7d90be9 ("clk-divider: Fix READ_ONLY when divider > 1")
> Reported-by: Zhang Qing <zhangqing@rock-chips.com>
> Signed-off-by: Heiko Stuebner <heiko@sntech.de>
> Reviewed-by: James Hogan <james.hogan@imgtec.com>
> ---

Hmph, the same patch[1] has been sitting in my inbox for months
now but I never got a response to my last question and then
forgot about it. I'll repeat the question here and hopefully we
can finish the discussion.

Maybe it would make more sense to have different ops for read
only dividers? The clk_set_rate op would be empty, while we would
have proper recalc_rate and round_rate ops. We can't just flat
out revert commit e6d5e7d90be9 (clk-divider: Fix READ_ONLY when
divider > 1, 2014-11-14) because that will introduce the problem
that it was fixing, but as long as we implement round_rate in
addition to recalc_rate it should work.

[1] http://lkml.kernel.org/r/1428392806-14538-1-git-send-email-jy0922.shim at samsung.com

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