linux-kernel - Re: [PATCH] PCI: Add cast when assigning PCI_ROM_ADDRESS

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Date:   Tue, 11 Apr 2017 12:08:38 +0200
From:   Mason <slash.tmp@...e.fr>
To:     Bjorn Helgaas <helgaas@...nel.org>,
        Matthias Kaehlcke <mka@...omium.org>
Cc:     linux-pci <linux-pci@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Grant Grundler <grundler@...omium.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Michael Davidson <md@...gle.com>
Subject: Re: [PATCH] PCI: Add cast when assigning PCI_ROM_ADDRESS_MASK to a
 32-bit variable

On 11/04/2017 03:24, Bjorn Helgaas wrote:

> On Mon, Apr 10, 2017 at 03:24:57PM -0700, Matthias Kaehlcke wrote:
>
>> This fixes a clang warning about "implicit conversion from 'unsigned
>> long' to 'u32'"
>>
>> Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
>> ---
>>  drivers/pci/probe.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>> index dfc9a2794141..148e80d5caf1 100644
>> --- a/drivers/pci/probe.c
>> +++ b/drivers/pci/probe.c
>> @@ -180,7 +180,7 @@ int __pci_read_base(struct pci_dev *dev, enum pci_bar_type type,
>>  	u16 orig_cmd;
>>  	struct pci_bus_region region, inverted_region;
>>  
>> -	mask = type ? PCI_ROM_ADDRESS_MASK : ~0;
>> +	mask = type ? (u32)PCI_ROM_ADDRESS_MASK : ~0;
> 
> Can we put the cast in the PCI_ROM_ADDRESS_MASK #define so we don't have to
> repeat it in all the uses?

Fixing these "implicit conversion" warnings, especially for
unsigned types, is a slippery slope. (The behavior of the
conversion is well-defined.)

How about changing the type of PCI_ROM_ADDRESS_MASK instead?
It's defined as ~0x7ffUL but it's only used in the context
of u32.

So make it an unsigned int:
#define PCI_ROM_ADDRESS_MASK (~0x7ffU)

AFAIU, unsigned int is 32 bits on all platforms supported
by Linux.

Regards.