linux-kernel - Re: [PATCH v1 0/2] PCI: ti: k3: Fix TI J721E PERST# polarity

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Message-ID: <ZpEGmeJUJxrDFQWa@ryzen.lan>
Date: Fri, 12 Jul 2024 12:34:01 +0200
From: Niklas Cassel <cassel@...nel.org>
To: Francesco Dolcini <francesco@...cini.it>
Cc: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Nishanth Menon <nm@...com>, Vignesh Raghavendra <vigneshr@...com>,
	Tero Kristo <kristo@...nel.org>, Conor Dooley <conor+dt@...nel.org>,
	Siddharth Vadapalli <s-vadapalli@...com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Francesco Dolcini <francesco.dolcini@...adex.com>,
	linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-omap@...r.kernel.org,
	linux-pci@...r.kernel.org
Subject: Re: [PATCH v1 0/2] PCI: ti: k3: Fix TI J721E PERST# polarity

On Thu, Jul 11, 2024 at 05:25:31PM +0200, Francesco Dolcini wrote:
> Hello Niklas,
> 
> On Thu, Jul 11, 2024 at 04:21:34PM +0200, Niklas Cassel wrote:
> > On Wed, Jul 03, 2024 at 12:00:34PM +0200, Francesco Dolcini wrote:
> > > From: Francesco Dolcini <francesco.dolcini@...adex.com>
> > > 
> > > Fix PCIe PERST# signal polarity in TI J721E used on TI K3 machines.
> > > 
> > > PCIe PERST# needs to be de-asserted for PCIe to work, however, the driver is
> > > doing the opposite and the device tree files are defining the signal with the
> > > wrong polarity to cope with that. Fix both the driver and the affected DT
> > > files.
> > 
> > While I understand why you want to fix this,
> > I'm not sure if you can actually do so without breaking device tree backwards
> > compatibility.
> 
> I understand this, and at the same time I know that this was done in the
> past for exactly the same reason, see for example commit 87620512681a
> ("PCI: apple: Fix PERST# polarity").

If you knew about it, I think that you should have stated that your are
breaking DT compatibility in the commit message, while also explaining it
is acceptable in your specific case.

I didn't know that there were other examples of drivers doing this.
Looking at your example, it seems that both:
1e33888fbe44 ("PCI: apple: Add initial hardware bring-up")
and
87620512681a ("PCI: apple: Fix PERST# polarity")

were first included in v5.16, so there was never a kernel release
with only one of the commits.

Anyway, I will eagerly await the DT maintainers feedback on this series.


Kind regards,
Niklas