Re: [PATCH] clk: tegra: Fix bypassing of PLLs

From: Jon Hunter <jonathanh@nvidia.com>
To: Tyler Baker <tyler.baker@linaro.org>,
	Thierry Reding <thierry.reding@gmail.com>
Cc: Peter De Schrijver <pdeschrijver@nvidia.com>,
	Prashant Gaikwad <pgaikwad@nvidia.com>,
	Michael Turquette <mturquette@baylibre.com>,
	"Stephen Boyd" <sboyd@codeaurora.org>,
	Stephen Warren <swarren@wwwdotorg.org>,
	Alexandre Courbot <gnurou@gmail.com>, <linux-clk@vger.kernel.org>,
	<linux-tegra@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Rhyland Klein <rklein@nvidia.com>,
	"Kevin's boot bot" <khilman@kernel.org>
Subject: Re: [PATCH] clk: tegra: Fix bypassing of PLLs
Date: Thu, 26 Nov 2015 09:56:44 +0000	[thread overview]
Message-ID: <5656D75C.9030203@nvidia.com> (raw)
In-Reply-To: <5655F489.6080808@nvidia.com>

On 25/11/15 17:48, Jon Hunter wrote:
> 
> On 25/11/15 15:52, Tyler Baker wrote:
>> On 25 November 2015 at 07:11, Thierry Reding <thierry.reding@gmail.com> wrote:
>>> On Mon, Nov 23, 2015 at 03:18:59PM -0800, Tyler Baker wrote:
>>>> Hi Jon,
>>>>
>>>> On 20 November 2015 at 07:11, Jon Hunter <jonathanh@nvidia.com> wrote:
>>>>> The _clk_disable_pll() function will attempt to place a PLL into bypass
>>>>> if the TEGRA_PLL_BYPASS is specified for the PLL and then disable the PLL
>>>>> by clearing the enable bit. To place the PLL into bypass, the bypass bit
>>>>> needs to be set and not cleared. Fix this by setting the bypass bit and
>>>>> not clearing it.
>>>>>
>>>>> Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
>>>>
>>>> The kernelci.org bot recently detected a jetson-tk1 boot failure[1][2]
>>>> in the tegra tree. This boot failure has only been observed when
>>>> booting with a multi_v7_defconfig kernel variant. The bot bisected[3]
>>>> this boot failure to this commit, and I confirmed reverting it on top
>>>> of the tegra for-next branch resolves the issue. The ramdisk[4] used
>>>> for booting is loaded with the modules from the build. It appears to
>>>> me that as the modules are being loaded in userspace by eudev the
>>>> jetson-tk1 locks up. I've sifted through the console logs a bit, and
>>>> found this splat to be most interesting[5].  Can you confirm this
>>>> issue on your end?
>>>
>>> Just to close the loop on this: we've discussed this on IRC and came to
>>> the conclusion that not using the bypass mode is safer (switching into
>>> and out of bypass can glitch). I've dropped this patch for now and Jon
>>> will be looking into a second revision of the patch which, in addition
>>> to fixing bypass (the fix is legit, it just happens to break because of
>>> the glitch, most likely), will also remove the BYPASS flag setting so
>>> that bypass will not be used.
>>
>> Thanks for the update, I appreciate you guys looking into this issue.
>> Please CC me on any fixes, I can re-test and give my tested-by.
> 
> No problem. On 2nd thoughts I am wondering if there is any value in
> bypassing the PLL when disabling it. I will look into this and see what
> I find out.

So I got some feedback saying that setting the bypass bit is not
glitch-less for most PLLs and so we should avoid setting this. It may
look a bit odd from reviewing the code, but it is clear to me know. So
we should just drop this change. However, if you like we could add a
comment to document why we are doing this.

Cheers
Jon