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From: Borislav Petkov <bp@alien8.de>
To: Saheed Bolarinwa <refactormyself@gmail.com>
Cc: Vignesh Raghavendra <vigneshr@ti.com>,
	trix@redhat.com, linux-fpga@vger.kernel.org,
	dri-devel@lists.freedesktop.org, linux-ide@vger.kernel.org,
	linux-mtd@lists.infradead.org, linux-i2c@vger.kernel.org,
	Miquel Raynal <miquel.raynal@bootlin.com>,
	linux-rdma@vger.kernel.org, Richard Weinberger <richard@nod.at>,
	linux-atm-general@lists.sourceforge.net, helgaas@kernel.org,
	linux-pci@vger.kernel.org, Jakub Kicinski <kuba@kernel.org>,
	linux-kernel-mentees@lists.linuxfoundation.org,
	Wolfgang Grandegger <wg@grandegger.com>,
	intel-gfx@lists.freedesktop.org, linux-gpio@vger.kernel.org,
	Marc Kleine-Budde <mkl@pengutronix.de>,
	skhan@linuxfoundation.org, bjorn@helgaas.com,
	Kalle Valo <kvalo@codeaurora.org>,
	linux-edac@vger.kernel.org, linux-hwmon@vger.kernel.org,
	netdev@vger.kernel.org, linux-wireless@vger.kernel.org,
	linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
	linux-crypto@vger.kernel.org, dmaengine@vger.kernel.org,
	"David S. Miller" <davem@davemloft.net>
Subject: Re: [RFC PATCH 00/17] Drop uses of pci_read_config_*() return value
Date: Sun, 2 Aug 2020 20:46:48 +0200
Message-ID: <20200802184648.GA23190@nazgul.tnic> (raw)
In-Reply-To: <b720aa44-895a-203b-e220-ecdb3acd9278@gmail.com>

On Sun, Aug 02, 2020 at 07:28:00PM +0200, Saheed Bolarinwa wrote:
> Because the value ~0 has a meaning to some drivers and only

No, ~0 means that the PCI read failed. For *every* PCI device I know.

Here's me reading from 0xf0 offset of my hostbridge:

# setpci -s 00:00.0 0xf0.l
01000000

That device doesn't have extended config space, so the last valid byte
is 0xff. Let's read beyond that:

# setpci -s 00:00.0 0x100.l
ffffffff

> Again, only the drivers can determine if ~0 is a valid value. This
> information is not available inside pci_config_read*().

Of course it is.

*every* change you've done in 6/17 - this is the only patch I have
received - checks for == ~0. So that check can just as well be moved
inside pci_config_read_*().

Here's how one could do it:

#define PCI_OP_READ(size, type, len) \
int noinline pci_bus_read_config_##size \
	(struct pci_bus *bus, unsigned int devfn, int pos, type *value)	\
{									\
	int res;							\
	unsigned long flags;						\
	u32 data = 0;							\
	if (PCI_##size##_BAD) return PCIBIOS_BAD_REGISTER_NUMBER;	\
	pci_lock_config(flags);						\
	res = bus->ops->read(bus, devfn, pos, len, &data);		\

	/* Check we actually read something which is not all 1s.*/
	if (data == ~0)
		return PCIBIOS_READ_FAILED;

	*value = (type)data;						\
	pci_unlock_config(flags);					\
	return res;							\
}

Also, I'd prefer a function to *not* return void but return either
an error or success. In the success case, the @value argument can be
consumed by the caller and otherwise not.

In any case, that change is a step in the wrong direction and I don't
like it, sorry.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette
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Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-01 11:24 Saheed O. Bolarinwa
2020-08-01 11:24 ` [RFC PATCH 15/17] iommu/vt-d: " Saheed O. Bolarinwa
2020-08-01 12:56 ` [RFC PATCH 00/17] " Borislav Petkov
2020-08-02 14:53   ` Tom Rix
2020-08-02 17:28   ` Saheed Bolarinwa
2020-08-02 18:46     ` Borislav Petkov [this message]
2020-08-02 19:14       ` Bjorn Helgaas
2020-08-02 20:18         ` Borislav Petkov
2020-08-03  6:56         ` Christoph Hellwig

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