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

From: Marc Zyngier <maz@kernel.org>
To: "Pali Rohár" <pali@kernel.org>
Cc: Luca Ceresoli <luca@lucaceresoli.net>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-pci@vger.kernel.org,
	kernel-team@android.com, Alyssa Rosenzweig <alyssa@rosenzweig.io>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Bjorn Helgaas <bhelgaas@google.com>
Subject: Re: [PATCH v2] PCI: apple: Follow the PCIe specifications when resetting the port
Date: Mon, 22 Nov 2021 14:43:37 +0000	[thread overview]
Message-ID: <87zgpw5jza.wl-maz@kernel.org> (raw)
In-Reply-To: <20211122120347.6qyiycqqjkgqvtta@pali>

On Mon, 22 Nov 2021 12:03:47 +0000,
Pali Rohár <pali@kernel.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@kernel.org>
> > Cc: Alyssa Rosenzweig <alyssa@rosenzweig.io>
> > Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> > Cc: Bjorn Helgaas <bhelgaas@google.com>
> > Cc: Pali Rohár <pali@kernel.org>
> 
> Looks good, but see comment below.
> 
> Acked-by: Pali Rohár <pali@kernel.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...

I guess that will be yet another thing to debug, but in the meantime
we have a reliable reset.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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