Re: [PATCH 2/3] ARM: iop32x: improve N2100 PCI broken parity quirk - Russell King

From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: Realtek linux nic maintainers <nic_swsd@realtek.com>,
	"linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Jakub Kicinski <kuba@kernel.org>,
	David Miller <davem@davemloft.net>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	Heiner Kallweit <hkallweit1@gmail.com>
Subject: Re: [PATCH 2/3] ARM: iop32x: improve N2100 PCI broken parity quirk
Date: Wed, 6 Jan 2021 00:50:53 +0000	[thread overview]
Message-ID: <20210106005053.GG1551@shell.armlinux.org.uk> (raw)
In-Reply-To: <20210106002833.GA1286114@bjorn-Precision-5520>

On Tue, Jan 05, 2021 at 06:28:33PM -0600, Bjorn Helgaas wrote:
> On Tue, Jan 05, 2021 at 10:42:31AM +0100, Heiner Kallweit wrote:
> >  {
> > -	if (dev->bus->number == 0 &&
> > -	    (dev->devfn == PCI_DEVFN(1, 0) ||
> > -	     dev->devfn == PCI_DEVFN(2, 0)))
> > -		dev->broken_parity_status = 1;
> > +	if (machine_is_n2100())
> > +		pci_quirk_broken_parity(dev);
> 
> Whatever "machine_is_n2100()" is (I can't find the definition), it is
> surely not equivalent to "00:01.0 || 00:02.0".  That change probably
> should be a separate patch with some explanation.

It isn't equivalent. It says "if this machine is N2100" which is a
completely different thing from matching the bus/devfn numbers.

You won't find a definition for machine_is_n2100() in the kernel;
it is generated from the machine table by scripts, along with lots
of similar definitions for each machine type:

/* The type of machine we're running on */
extern unsigned int __machine_arch_type;

#ifdef CONFIG_MACH_N2100
# ifdef machine_arch_type
#  undef machine_arch_type
#  define machine_arch_type     __machine_arch_type
# else
#  define machine_arch_type     MACH_TYPE_N2100
# endif
# define machine_is_n2100()     (machine_arch_type == MACH_TYPE_N2100)
#else
# define machine_is_n2100()     (0)
#endif

The upshot of the above is that machine_is_n2100() is constant zero
if the platform is not configured (thereby allowing the compiler to
eliminate the code.) If it is the _only_ platform selected, then it
evaluates to an always-true expression. Otherwise, it becomes a
run-time evaluated conditional.

We may have better ways to do this in modern kernels, but this was
invented decades ago, and works with zero runtime overhead.

> If this makes the quirk safe to use in a generic kernel, that sounds
> like a good thing.
> 
> I guess a parity problem could be the result of a defect in either the
> device (e.g., 0x8169), which would be an issue in *all* platforms, or
> a platform-specific issue in the way it's wired up.  I assume it's the
> latter because the quirk is not in drivers/pci/quirks.c.
> 
> Why is it safe to restrict this to device ID 0x8169?  If this is
> platform issue, it might affect any device in the slot.

You assume the platform has multiple PCI slots - it doesn't. It's an
embedded platform (it's sold as a NAS) and it has a single mini-PCI
slot for a WiFi card. Without that populated, lspci -n looks like
this:

00:01.0 0200: 10ec:8169 (rev 10)
00:02.0 0200: 10ec:8169 (rev 10)
00:03.0 0180: 1095:3512 (rev 01)
00:04.0 0c03: 1106:3038 (rev 61)
00:04.1 0c03: 1106:3038 (rev 61)
00:04.2 0c03: 1106:3104 (rev 63)

Where all those devices are soldered to the board.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!

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