linux-kernel - Re: [PATCH 7/7] PCI: apple: Add T602x PCIe support

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Message-ID: <86o6z4srro.wl-maz@kernel.org>
Date: Fri, 14 Feb 2025 11:13:47 +0000
From: Marc Zyngier <maz@...nel.org>
To: "Sven Peter" <sven@...npeter.dev>
Cc: "Alyssa Rosenzweig" <alyssa@...enzweig.io>,	"Hector Martin"
 <marcan@...can.st>,	"Bjorn Helgaas" <bhelgaas@...gle.com>,
	"Lorenzo Pieralisi" <lpieralisi@...nel.org>,	Krzysztof Wilczyński <kw@...ux.com>,	"Manivannan Sadhasivam"
 <manivannan.sadhasivam@...aro.org>,	"Rob Herring" <robh@...nel.org>,
	"Krzysztof Kozlowski" <krzk+dt@...nel.org>,	"Conor Dooley"
 <conor+dt@...nel.org>,	"Mark Kettenis" <kettenis@...nbsd.org>,
	"Stan Skowronek" <stan@...ellium.com>,	asahi@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,	linux-pci@...r.kernel.org,
	devicetree@...r.kernel.org,	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/7] PCI: apple: Add T602x PCIe support

On Thu, 13 Feb 2025 19:51:31 +0000,
"Sven Peter" <sven@...npeter.dev> wrote:
> 
> Hi,
> 
> On Wed, Feb 12, 2025, at 10:55, Marc Zyngier wrote:
> > On Tue, 11 Feb 2025 19:54:32 +0000,
> > Alyssa Rosenzweig <alyssa@...enzweig.io> wrote:
> >> 
> >> From: Hector Martin <marcan@...can.st>
> >> 
> >> This version of the hardware moved around a bunch of registers, so we
> >> drop the old compatible for these and introduce register offset
> >> structures to handle the differences.
> >> 
> >> Signed-off-by: Hector Martin <marcan@...can.st>
> >> Signed-off-by: Alyssa Rosenzweig <alyssa@...enzweig.io>
> >> ---
> >>  drivers/pci/controller/pcie-apple.c | 125 ++++++++++++++++++++++++++++++------
> >>  1 file changed, 105 insertions(+), 20 deletions(-)
> >> 
> >> diff --git a/drivers/pci/controller/pcie-apple.c b/drivers/pci/controller/pcie-apple.c
> >> index 7f4839fb0a5b15a9ca87337f53c14a1ce08301fc..7c598334427cb56ca066890ac61143ae1d3ed744 100644
> ...
> >
> >> +	else
> >> +		rmw_set(PHY_LANE_CFG_REFCLKCGEN, port->phy + PHY_LANE_CFG);
> >> +	rmw_clear(PORT_APPCLK_CGDIS, port->base + PORT_APPCLK);
> >> +
> >
> > Can you elaborate on this particular change?
> >
> > I always assumed this was some clock-gating that needed to occur
> > *before* the link training was started. This is now taking place after
> > training, and the commit message doesn't say anything about it.
> 
> It's been a while but as far as I can tell APPCLK seems to be related
> to the IOMMUs attached to this controller. If it's disabled all reads
> from the respective IOMMU MMIO either came back as 0xffff.. or SError
> (don't remember which one it was) but pcie itself worked just fine
> (until any device tried DMA ofc).
> 
> At least on M1 this entire sequence only works because we already
> setup PORT_APPCLK_EN inside m1n1. If we didn't do this (like e.g
> for the thunderbolt pcie/dart) the DART probe would already fail.

OK, so the exact location of this particular write doesn't matter as
long as it happens before we start enabling a device on that port.

I'm still perplexed by this one though:

+	if (pcie->hw->port_refclk)
+		rmw_clear(PORT_REFCLK_CGDIS, port->base + PORT_REFCLK);
+	else
+		rmw_set(PHY_LANE_CFG_REFCLKCGEN, port->phy + PHY_LANE_CFG);

which looks like it switches on the reference clock for the port. I
have a very vague recollection that it was required early before
m1n1/u-boot grew some PCI initialisation (yes, a long while ago).

Thanks,

	M.

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