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

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Message-Id: <ae7bdc0c-c691-43a7-8cd7-b1c22c7623c0@app.fastmail.com>
Date: Thu, 13 Feb 2025 20:51:31 +0100
From: "Sven Peter" <sven@...npeter.dev>
To: "Marc Zyngier" <maz@...nel.org>,
 "Alyssa Rosenzweig" <alyssa@...enzweig.io>
Cc: "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

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.



Sven