linux-kernel - Re: [PATCH] drivers/char/mem.c: Add /dev/ioports, supporting 16-bit and 32-bit ports

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Date:	Tue, 29 Dec 2015 21:23:19 +0530
From:	Santosh Shukla <santosh.shukla@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>, aalex.williamson@...hat.com
Cc:	"H. Peter Anvin" <hpa@...or.com>, josh@...htriplett.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	akpm@...ux-foundation.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-api@...r.kernel.org, yuanhan.liu@...ux.intel.com,
	Santosh Shukla <sshukla@...sta.com>
Subject: Re: [PATCH] drivers/char/mem.c: Add /dev/ioports, supporting 16-bit
 and 32-bit ports

On 29 December 2015 at 18:58, Arnd Bergmann <arnd@...db.de> wrote:
> On Wednesday 23 December 2015 17:04:40 Santosh Shukla wrote:
>> On 23 December 2015 at 03:26, Arnd Bergmann <arnd@...db.de> wrote:
>> > On Tuesday 22 December 2015, Santosh Shukla wrote:
>> >> }
>> >>
>> >> So I care for /dev/ioport types interface who could do more than byte
>> >> data copy to/from user-space. I tested this patch with little
>> >> modification and could able to run pmd driver for arm/arm64 case.
>> >>
>> >> Like to know how to address pci_io region mapping problem for
>> >> arm/arm64, in-case /dev/ioports approach is not acceptable or else I
>> >> can spent time on restructuring the patch?
>> >>
>> >
>> > For the use case you describe, can't you use the vfio framework to
>> > access the PCI BARs?
>> >
>>
>> I looked at file: drivers/vfio/pci/vfio_pci.c, func vfio_pci_map() and
>> it look to me that it only maps ioresource_mem pci region, pasting
>> code snap:
>>
>> if (!(pci_resource_flags(pdev, index) & IORESOURCE_MEM))
>> return -EINVAL;
>> ....
>>
>> and I want to map ioresource_io pci region for arm platform in my
>> use-case. Not sure vfio maps pci_iobar region?
>
> Mapping I/O BARs is not portable, notably it doesn't work on x86.
>
> You should be able access them using the read/write interface on
> the vfio device.
>
Right, x86 doesn't care as iopl() could give userspace application
direct access to ioports.

Also, Alex in other dpdk thread [1] suggested someone to propose io
bar mapping in vfio-pci, I guess in particular to non-x86 arch so I
started working on it.

Thanks.

[1] http://dpdk.org/ml/archives/dev/2015-December/030852.html

>         Arnd
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