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SubjectRe: [PATCH] clk: aspeed: Prevent reset if clock is enabled
Hi Eddie,

On Thu, Mar 8, 2018 at 3:06 AM, Eddie James <eajames@linux.vnet.ibm.com> wrote:
> According to the Aspeed specification, the reset and enable sequence
> should be done when the clock is stopped. The specification doesn't
> define behavior if the reset is done while the clock is enabled.
>
> From testing on the AST2500, the LPC Controller has problems if the
> clock is reset while enabled.
>
> Therefore, check whether the clock is enabled or not before performing
> the reset and enable sequence in the Aspeed clock driver.
>
> Signed-off-by: Eddie James <eajames@linux.vnet.ibm.com>
> ---
> drivers/clk/clk-aspeed.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/drivers/clk/clk-aspeed.c b/drivers/clk/clk-aspeed.c
> index 9f7f931..a13054d 100644
> --- a/drivers/clk/clk-aspeed.c
> +++ b/drivers/clk/clk-aspeed.c
> @@ -212,6 +212,12 @@ static int aspeed_clk_enable(struct clk_hw *hw)
> u32 clk = BIT(gate->clock_idx);
> u32 rst = BIT(gate->reset_idx);
> u32 enval;
> + u32 reg;
> +
> + /* Only reset/enable/unreset if clock is stopped */
> + regmap_read(gate->map, ASPEED_CLK_STOP_CTRL, &reg);
> + if (!(reg & clk))
> + return 0;

This doesn't generalise to all of the clocks, as some clocks use set
to disable. Perhaps we could do something like this:

/* Only reset/enable/unreset if clock is stopped. The LPC clock
on ast2500 has issues otherwise */
enval = (gate->flags & CLK_GATE_SET_TO_DISABLE) ? 0 : clk;
regmap_read(gate->map, ASPEED_CLK_STOP_CTRL, &reg);
if ((reg & clk) == enval) {
spin_unlock_irqrestore(gate->lock, flags);
return 0;
}

I think we should also do this operation under the lock.

Please cc Ryan Chen <ryan_chen@aspeedtech.com> so he can confirm that
this workaround is valid, and credit Lei who spent a lot of time
investigating this issue. Perhaps "Root-caused-by".

Cheers,

Joel

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