linux-kernel - Re: [PATCH v2] PCI: apple: Follow the PCIe specifications when resetting the port

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Date:   Mon, 22 Nov 2021 22:50:48 +0100 (CET)
From:   Mark Kettenis <mark.kettenis@...all.nl>
To:     Marc Zyngier <maz@...nel.org>
Cc:     pali@...nel.org, luca@...aceresoli.net,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-pci@...r.kernel.org, kernel-team@...roid.com,
        alyssa@...enzweig.io, lorenzo.pieralisi@....com,
        bhelgaas@...gle.com, joey.gouly@....com
Subject: Re: [PATCH v2] PCI: apple: Follow the PCIe specifications when
 resetting the port

> Date: Mon, 22 Nov 2021 14:43:37 +0000
> From: Marc Zyngier <maz@...nel.org>
> 
> On Mon, 22 Nov 2021 12:03:47 +0000,
> Pali Rohár <pali@...nel.org> wrote:
> > 
> > On Monday 22 November 2021 10:41:56 Marc Zyngier wrote:
> > > While the Apple PCIe driver works correctly when directly booted
> > > from the firmware, it fails to initialise when the kernel is booted
> > > from a bootloader using PCIe such as u-boot.
> > > 
> > > That's beacuse we're missing a proper reset of the port (we only
> > > clear the reset, but never assert it).
> > > 
> > > The PCIe spec requirements are two-fold:
> > > 
> > > - #PERST must be asserted before setting up the clocks, and
> > >   stay asserted for at least 100us (Tperst-clk).
> > > 
> > > - Once #PERST is deasserted, the OS must wait for at least 100ms
> > >   "from the end of a Conventional Reset" before we can start talking
> > >   to the devices
> > > 
> > > Implementing this results in a booting system.
> > > 
> > > Fixes: 1e33888fbe44 ("PCI: apple: Add initial hardware bring-up")
> > > Signed-off-by: Marc Zyngier <maz@...nel.org>
> > > Cc: Alyssa Rosenzweig <alyssa@...enzweig.io>
> > > Cc: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
> > > Cc: Bjorn Helgaas <bhelgaas@...gle.com>
> > > Cc: Pali Rohár <pali@...nel.org>
> > 
> > Looks good, but see comment below.
> > 
> > Acked-by: Pali Rohár <pali@...nel.org>
> 
> Thanks for that.
> 
> > 
> > > ---
> > >  drivers/pci/controller/pcie-apple.c | 10 ++++++++++
> > >  1 file changed, 10 insertions(+)
> > > 
> > > diff --git a/drivers/pci/controller/pcie-apple.c b/drivers/pci/controller/pcie-apple.c
> > > index 1bf4d75b61be..957960a733c4 100644
> > > --- a/drivers/pci/controller/pcie-apple.c
> > > +++ b/drivers/pci/controller/pcie-apple.c
> > > @@ -539,13 +539,23 @@ static int apple_pcie_setup_port(struct apple_pcie *pcie,
> > >  
> > >  	rmw_set(PORT_APPCLK_EN, port->base + PORT_APPCLK);
> > >  
> > > +	/* Engage #PERST before setting up the clock */
> > > +	gpiod_set_value(reset, 0);
> > > +
> > >  	ret = apple_pcie_setup_refclk(pcie, port);
> > >  	if (ret < 0)
> > >  		return ret;
> > >  
> > > +	/* The minimal Tperst-clk value is 100us (PCIe CMS r2.0, 2.6.2) */
> > > +	usleep_range(100, 200);
> > > +
> > > +	/* Deassert #PERST */
> > >  	rmw_set(PORT_PERST_OFF, port->base + PORT_PERST);
> > >  	gpiod_set_value(reset, 1);
> > 
> > + Luca
> > 
> > Just one comment. PERST# (PCIe Reset) is active-low signal. De-asserting
> > means to really set value to 1.
> > 
> > But there was a discussion that de-asserting should be done by call:
> >   gpiod_set_value(reset, 0);
> > 
> > https://lore.kernel.org/linux-pci/51be082a-ff10-8a19-5648-f279aabcac51@lucaceresoli.net/
> > 
> > Could we make this new pcie-apple.c driver to use gpiod_set_value(reset, 0)
> > for de-asserting, like in other drivers?
> 
> I guess it depends whether you care about the assertion or the signal
> itself. I think we may have a bug in the way the GPIOs are handled at
> the moment, as it makes no difference whether I register the GPIO are
> active high or active low...

That's unfortunate.  But maybe that's an opportunity to fix the
devicetree to use GPIO_ACTIVE_LOW for these GPIOs?