linux-kernel - Re: [PATCH v1] clk: qcom: lpass: Add CLK_IGNORE

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Date:   Thu, 17 Jan 2019 16:49:22 +0530
From:   Taniya Das <tdas@...eaurora.org>
To:     Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>
Cc:     Andy Gross <andy.gross@...aro.org>,
        David Brown <david.brown@...aro.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        linux-arm-msm@...r.kernel.org, linux-soc@...r.kernel.org,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] clk: qcom: lpass: Add CLK_IGNORE_UNUSED for lpass
 clocks



On 1/15/2019 3:55 AM, Stephen Boyd wrote:
> Quoting Taniya Das (2019-01-13 22:12:39)
>>
>>
>> On 1/8/2019 2:34 AM, Stephen Boyd wrote:
>>>
>>> As far as I know, I'm not suggesting the use of CLK_IS_CRITICAL here.
>>> But removing CLK_IS_CRITICAL and relying on some random bootloader
>>> behavior also looks wrong. Can you clarify what's going on?
>>>
>>
>> To enable LPASS clocks the requirement is to enable the GCC_LPASS_SWAY
>> clock.
>> 1) If the LPASS drivers are enabled/probed before the clock late init
>> the client would take care to maintain the dependency to enable the
>> GCC_LPASS_SWAY clock before enabling the LPASS clocks.
>>
>> 2) There could be a condition where the LPASS drivers would probe/init
>> later the clock late_init. When the clock_late_init would try to access
>> the LPASS clocks, since we cannot maintain the dependency this access
>> would fail. To avoid this the earlier patch has made the GCC_LPASS_SWAY
>> clock as CRITICAL.
>>
>> 3) Marking the GCC_LPASS_SWAY clock as CRITICAL has a issue, in the case
>> where the LPASS subsystem would be restarted due to some critical
>> failure on LPASS. Toggling the restart register of LPASS would clear the
>> hardware state of this clock and thus the next access of the LPASS
>> clocks would result in failure of the system.
>>
>> 4) To avoid issues happening in (2) and (3) all the LPASS clocks chould
>> be safely marked as CLK_IGNORE_UNUSED. And lpass drivers would take care
>> of the dependency to enable the required clocks.
>>
> 
> Ok, so why can't we enable/disable the lpass sway clk in the
> prepare/unprepare phase of the lpass clk driver paths? Or why can't we
> forcibly enable this lpass sway clk after the reset is deasserted? Which
> clk controller is the reset part of? GCC or LPASS?

It is part of Always On Subsystem.

> 
> It still sounds like the LPASS clk driver isn't handling dependencies it
> has on accessing registers, but maybe we can get away with not handling
> the dependency still if we make the reset "do the right thing" and turn
> the clk back on so it stays "critical".
>

This is a reset from hardware and it does not bring back the clock to 
the previous state and so we can not mark it "critical". I would submit 
the next series with comments updated. Please let me know in case you 
have any comments.


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