Re: [PATCH v6] clk: qcom: Add lpass clock controller driver for SDM845

From: Taniya Das
Date: Wed Oct 17 2018 - 08:04:20 EST

On 10/17/2018 5:07 PM, Taniya Das wrote:
Hello Stephen,

On 10/12/2018 11:05 PM, Stephen Boyd wrote:
Quoting Taniya Das (2018-10-09 23:12:27)

On 10/10/2018 2:22 AM, Stephen Boyd wrote:
Quoting Taniya Das (2018-10-09 10:26:38)
Hello Stephen,

On 10/8/2018 8:14 AM, Stephen Boyd wrote:
Quoting Taniya Das (2018-10-04 05:02:26)
Add support for the lpass clock controller found on SDM845 based devices.
This would allow lpass peripheral loader drivers to control the clocks to
bring the subsystem out of reset.
LPASS clocks present on the global clock controller would be registered
with the clock framework based on the device tree flag. Also do not gate
these clocks if they are left unused.

Why not gate them? This statement states what the code is doing, not why
it's doing it which is the more crucial information that should be
described in the commit text. Also, please add a comment about it to the
code next to the flag.

I am concerned that it doesn't make any sense though, so probably it
shouldn't be marked as CLK_IGNORE_UNUSED and it's papering over some
other larger bug that needs to be fixed.

It does not have any bug, it is just that to access these lpass
registers we would need the GCC lpass registers to be enabled. I would
update the same in the commit text.

During clock late_init these clocks should not be accessed to check the
clock status as they would result in unclocked access. The client would
request these clocks in the correct order and it would not have any issue.

That seems like the bug right there. If the LPASS registers can't be
accessed unless the clks in GCC are enabled then this driver needs to
turn the clks on before reading/writing registers. Marking the clks as
ignore unused is skipping around the real problem.

If the driver requests for the clocks they would maintain the order. But
if the clock late init call is invoked before the driver requests, there
is no way I could manage this dependency, that is the only reason to
mark them unused.

Which driver are we talking about here? The lpass clk driver? Presumably
the lpass clk driver would request the GCC clks and turn them on in
probe and then register any lpass clks. If the lpass clk driver probes
bfeore late init, then the gcc clks will be enabled and everything
works, and if the lpass clk driver probes after late init then the clks
that can't be touched without gcc clks enabled won't be registered, and
then they won't be touched. What goes wrong?

Okay, sure, I will take the GCC clock handles and then enable/disable them accordingly.

I missed earlier, so here is what you suggest

gcc_probe --> GCC LPASS clocks registered.
lpass_probe --> clk_get on gcc_lpass_clocks/ clk_prepare_enable -->
register the lpass clocks --> clk_disable_unprepare gcc_lpass_clocks.

But the problem is not during the above. It is the below
static void clk_disable_unused_subtree(struct clk_core *core)

if (clk_core_is_enabled(core)) { --> This access fails.


Please let me know your comments in case I missed something.
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