linux-kernel - Re: [PATCH 08/13] phy: qcom-qmp-pcie: drop power-down delay config

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Message-ID: <dbd85c78-86dc-8dd0-83d2-43af933e5a92@linaro.org>
Date:   Tue, 11 Oct 2022 17:41:28 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     Johan Hovold <johan+linaro@...nel.org>,
        Vinod Koul <vkoul@...nel.org>, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Kishon Vijay Abraham I <kishon@...com>,
        linux-arm-msm@...r.kernel.org, linux-phy@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/13] phy: qcom-qmp-pcie: drop power-down delay config

On 11/10/2022 17:17, Johan Hovold wrote:
> On Tue, Oct 11, 2022 at 05:04:04PM +0300, Dmitry Baryshkov wrote:
>> On 11/10/2022 16:53, Johan Hovold wrote:
>>> On Tue, Oct 11, 2022 at 04:46:53PM +0300, Dmitry Baryshkov wrote:
>>>> On 11/10/2022 16:14, Johan Hovold wrote:
>>>>> The power-down delay was included in the first version of the QMP driver
>>>>> as an optional delay after powering on the PHY (using
>>>>> POWER_DOWN_CONTROL) and just before starting it. Later changes modified
>>>>> this sequence by powering on before initialising the PHY, but the
>>>>> optional delay stayed where it was (i.e. before starting the PHY).
>>>>>
>>>>> The vendor driver does not use a delay before starting the PHY and this
>>>>> is likely not needed on any platform unless there is a corresponding
>>>>> delay in the vendor kernel init sequence tables (i.e. in devicetree).
>>>>>
>>>>> Let's keep the delay for now, but drop the redundant delay period
>>>>> configuration while increasing the unnecessarily low timer slack
>>>>> somewhat.
>>>>
>>>> Actually, the vendor driver does this 995..1005 sleep. But contrary to
>>>> our driver it does that after programming whole PHY init sequence, which
>>>> includes SW_RESET / START_CTL, but before programming the pipe clocks.
>>>
>>> Right, it does it after starting the PHY which means that you don't have
>>> to poll for as long for the PHY status.
>>>
>>> It's a different delay entirely.
>>
>> No-no-no. The 995-1005 delay was added guess for which SoC? For ipq8074,
>> where the config tables contain the ugly CFG_L writes for SW_RESET /
>> START_CTRL. So, it is the same delay, but added by somebody who didn't
>> care enough. The original 10-11 delay is a completely different story,
>> you are correct here.
> 
> Yeah, I noticed that ipq8074 was the first to abuse the prwdn_delay
> and possibly because of it starting the PHY already in its PCS table
> (which it never should have).
> 
> I'm talking about the intent of pwrdn_delay which was to add a delay
> after powering-on the phy and before starting it.
> 
> The vendor driver has a 1 ms delay after starting the PHY and before it
> starts polling as the PHY on newer SoC tend to take > 1 ms before they
> are ready.
> 
> So, I still claim that that delay in the vendor driver is a different
> one entirely.
> 
>> Thus, I'd say, the PCIe delay should be moved after the registers
>> programming.
> 
> No, not necessarily. Again, that's an optimisation in the vendor driver
> to avoid polling so many times. Since I can say for sure that there are
> no PHY that start in less than 1 ms, I wouldn't add it unconditionally.

I don't think it's an optimization. For me it looks like some kind of 
stabilization delay before touching pipe clocks.

> 
> Either way, separate change.
>   
>>>> I think we can either drop this delay completely, or move it before
>>>> read_poll_timeout().
>>>
>>> It definitely shouldn't be used for any new platforms, but I opted for
>>> the conservative route of keeping it in case some of the older platforms
>>> actually do need it.
>>>
>>> My bet is that this is all copy-paste cruft that could be removed, but
>>> I'd rather do that as a separate follow-on change. Perhaps after testing
>>> some more SoC after removing the delay.
>>>
>>> SC8280XP certainly doesn't need it.
>>
>> I think in our case this delay just falls into status polling. We'd
>> probably need it, if we'd add the noretain handling.
> 
> I'm not sure I understand what you're referring to here ("noretain
> handling")?

 From what I see in the downstream (4.19 at hand), the sequence is the 
following:

program_phy_config() // including SW_RESET & START_CTRL

delay

for pipe clocks:
clk_set_flags(info->hdl, CLKFLAG_NORETAIN_MEM)
clk_set_flags(info->hdl, CLKFLAG_NORETAIN_PERIPH)

set clock rates, prepare & enable pipe clocks

wmb()

poll for the PHY STATUS


-- 
With best wishes
Dmitry