linux-kernel - RE: [PATCH 3/3] clk: qcom: Set BRANCH_HALT

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Date:   Mon, 14 Nov 2016 09:21:24 +0530
From:   "Sricharan" <sricharan@...eaurora.org>
To:     "'Stephen Boyd'" <sboyd@...eaurora.org>
Cc:     "'Rajendra Nayak'" <rnayak@...eaurora.org>,
        <mturquette@...libre.com>, <linux-clk@...r.kernel.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <stanimir.varbanov@...aro.org>
Subject: RE: [PATCH 3/3] clk: qcom: Set BRANCH_HALT_DELAY flags for venus core0/1 clks

Hi Stephen,

>>
>> So the above is the sequence which is actually carried out on the
>> firmware side. The same can be done in host as well.
>> The clocks stuck issue indeed is not there with this.
>
>Great! We've finally connected on what the actual problem is.
>
>> But with the above sequence we need to add a step to do inverse
>> of STEP3 above (ie write the registers to de-assert hw_signal),
>> to keep the subdomains in off, till firmware uses it. So the
>> above sequence helps to avoid masking the halt check, although
>> the host really does not wants to use these clocks, except
>> setting it up for the firmware.
>>
>
>Right, but knowing that the clocks failed to turn on in the first
>place is much safer than silently ignoring the failure.
>Otherwise, we could hand over control to the firmware, and the
>firmware would fail to operate the hardware, and we're stuck with
>debugging the firmware now. That sounds quite painful to figure
>out.
>

Right, i already debugged this sort of a scenario which was quite
paintful sometime back :)

>If we properly toggle the video hw bits in coordination with
>firmware and turn on/off the clocks with the GDSC ON, then
>debugging is made simpler. The point is, we don't want to lose
>robustness by silencing halt checks. The semantics of
>clk_enable() means the clock is running, and that won't be true
>here unless we ensure the GDSC is enabled.
>

ok, which means with this approach, this patch can be dropped and
the other bits needs to be added to the video driver. I will follow that
up with Stanimir in his video driver patches.

Regards,
 Sricharan