From: Bjorn Helgaas <helgaas@kernel.org>
To: Amey Narkhede <ameynarkhede03@gmail.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>,
linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
Leon Romanovsky <leon@kernel.org>
Subject: Re: [PATCH v2] PCI: Check value of resource alignment before using __ffs
Date: Tue, 25 May 2021 17:01:15 -0500 [thread overview]
Message-ID: <20210525220115.GA1233963@bjorn-Precision-5520> (raw)
In-Reply-To: <20210422105538.76057-1-ameynarkhede03@gmail.com>
On Thu, Apr 22, 2021 at 04:25:38PM +0530, Amey Narkhede wrote:
> Return value of __ffs is undefined if no set bit exists in
> its argument. This indicates that the associated BAR has
> invalid alignment.
>
> Signed-off-by: Amey Narkhede <ameynarkhede03@gmail.com>
> ---
> drivers/pci/setup-bus.c | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> index 2ce636937c6e..ce5380bdd2fd 100644
> --- a/drivers/pci/setup-bus.c
> +++ b/drivers/pci/setup-bus.c
> @@ -1044,10 +1044,11 @@ static int pbus_size_mem(struct pci_bus *bus, unsigned long mask,
> * resources.
> */
> align = pci_resource_alignment(dev, r);
> - order = __ffs(align) - 20;
> - if (order < 0)
> - order = 0;
> - if (order >= ARRAY_SIZE(aligns)) {
> + if (align) {
> + order = __ffs(align) - 20;
> + order = (order < 0) ? 0 : order;
> + }
> + if (!align || order >= ARRAY_SIZE(aligns)) {
> pci_warn(dev, "disabling BAR %d: %pR (bad alignment %#llx)\n",
> i, r, (unsigned long long) align);
> r->flags = 0;
I know this is solving a theoretical problem. Is it also solving a
*real* problem?
I dislike the way it complicates the code and the usage of "align" and
"order". I know that when "!align", we don't evaluate the
"order >= ARRAY_SIZE()" (which would involve an uninitialized value),
but it just seems ugly, and I'm not sure how much we benefit.
And the "disabling BAR" part is gross. I know you're not changing
that part, but it's just wrong. Setting r->flags = 0 certainly does
not disable the BAR. It might make Linux ignore it, but that doesn't
mean the hardware ignores it. When we turn on PCI_COMMAND_MEMORY, the
BAR is enabled along with all the other memory BARs.
Bjorn
next prev parent reply other threads:[~2021-05-25 22:01 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-22 10:55 [PATCH v2] PCI: Check value of resource alignment before using __ffs Amey Narkhede
2021-04-22 10:57 ` Leon Romanovsky
2021-05-24 20:32 ` Amey Narkhede
2021-05-25 22:01 ` Bjorn Helgaas [this message]
2021-05-26 9:06 ` Amey Narkhede
2021-05-28 0:04 ` Bjorn Helgaas
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