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

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Date:   Tue, 11 Oct 2022 17:04:04 +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 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.

Thus, I'd say, the PCIe delay should be moved after the registers 
programming.

> 
>> 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.

> 
> Johan

-- 
With best wishes
Dmitry