linux-kernel - Re: [RESEND PATCH v2] PCI: mediatek: Delay 100ms to wait power and clock to become stable

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Message-ID: <20211207180140.GA5938@lpieralisi>
Date:   Tue, 7 Dec 2021 18:01:48 +0000
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     qizhong cheng <qizhong.cheng@...iatek.com>,
        Ryder Lee <ryder.lee@...iatek.com>,
        Jianjun Wang <jianjun.wang@...iatek.com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, chuanjia.liu@...iatek.com,
        Pali Rohár <pali@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Mark Kettenis <mark.kettenis@...all.nl>,
        Luca Ceresoli <luca@...aceresoli.net>
Subject: Re: [RESEND PATCH v2] PCI: mediatek: Delay 100ms to wait power and
 clock to become stable

On Tue, Dec 07, 2021 at 11:54:16AM -0600, Bjorn Helgaas wrote:
> [+cc Marc, Alyssa, Mark, Luca for reset timing questions]
> 
> On Tue, Dec 07, 2021 at 04:41:53PM +0800, qizhong cheng wrote:
> > Described in PCIe CEM specification sections 2.2 (PERST# Signal) and
> > 2.2.1 (Initial Power-Up (G3 to S0)). The deassertion of PERST# should
> > be delayed 100ms (TPVPERL) for the power and clock to become stable.
> > 
> > Signed-off-by: qizhong cheng <qizhong.cheng@...iatek.com>
> > Acked-by: Pali Rohï¿½r <pali@...nel.org>
> > ---
> > 
> > v2:
> >  - Typo fix.
> >  - Rewrap into one paragraph.
> 
> 1) If you change something, even in the commit log or comments, it is
> a new version, not a "RESEND".  A "RESEND" means "I sent this quite a
> while ago and didn't hear anything, so I'm sending the exact same
> thing again in case the first one got lost."
> 
> 2) I suggested a subject line update, which apparently got missed.
> Here's a better one:
> 
>   PCI: mediatek: Assert PERST# for 100ms for power and clock to stabilize
> 
> 3) Most importantly, this needs to be reconciled with the similar
> change to the apple driver:
> 
>   https://lore.kernel.org/r/20211123180636.80558-2-maz@kernel.org
> 
> In the apple driver, we're doing:
> 
>   - Assert PERST#
>   - Set up REFCLK
>   - Sleep 100us (T_perst-clk, CEM r5 2.2, 2.9.2)
>   - Deassert PERST#
>   - Sleep 100ms (not sure there's a name? PCIe r5 6.6.1)
> 
> But here in mediatek, we're doing:
> 
>   - Assert PERST#
>   - Sleep 100ms (T_pvperl, CEM r5 2.2, 2.2.1, 2.9.2)
>   - Deassert PERST#
> 
> My questions:
> 
>   - Where does apple enforce T_pvperl?  I can't tell where power to
>     the slot is turned on.
> 
>   - Where does mediatek enforce the PCIe sec 6.6.1 delay after
>     deasserting PERST# and before config requests?
> 
>   - Does either apple or mediatek support speeds greater than 5 GT/s,
>     and if so, shouldn't we start the sec 6.6.1 100ms delay *after*
>     Link training completes?

I dropped this patch from my pci/mediatek branch, waiting for
clarification.

Thanks,
Lorenzo

> >  drivers/pci/controller/pcie-mediatek.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/drivers/pci/controller/pcie-mediatek.c b/drivers/pci/controller/pcie-mediatek.c
> > index 2f3f974977a3..a61ea3940471 100644
> > --- a/drivers/pci/controller/pcie-mediatek.c
> > +++ b/drivers/pci/controller/pcie-mediatek.c
> > @@ -702,6 +702,13 @@ static int mtk_pcie_startup_port_v2(struct mtk_pcie_port *port)
> >  	 */
> >  	writel(PCIE_LINKDOWN_RST_EN, port->base + PCIE_RST_CTRL);
> >  
> > +	/*
> > +	 * Described in PCIe CEM specification sections 2.2 (PERST# Signal) and
> > +	 * 2.2.1 (Initial Power-Up (G3 to S0)). The deassertion of PERST# should
> > +	 * be delayed 100ms (TPVPERL) for the power and clock to become stable.
> > +	 */
> > +	msleep(100);
> > +
> >  	/* De-assert PHY, PE, PIPE, MAC and configuration reset	*/
> >  	val = readl(port->base + PCIE_RST_CTRL);
> >  	val |= PCIE_PHY_RSTB | PCIE_PERSTB | PCIE_PIPE_SRSTB |
> > -- 
> > 2.25.1
> >