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

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Message-ID: <CAAyOgsYuMWudLgbSNM+QAkSXpU_fp6Ue01r=Ufmf5MXUHMs4UQ@mail.gmail.com>
Date:	Tue, 29 Dec 2015 22:00:43 +0530
From:	Santosh Shukla <sshukla@...sta.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Santosh Shukla <santosh.shukla@...aro.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	"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 <yuanhan.liu@...ux.intel.com>
Subject: Re: [PATCH] drivers/char/mem.c: Add /dev/ioports, supporting 16-bit
 and 32-bit ports

On Tue, Dec 29, 2015 at 9:50 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Tuesday 29 December 2015 21:25:15 Santosh Shukla wrote:
>> mistakenly added wrong email-id of alex, looping his correct one.
>>
>> On 29 December 2015 at 21:23, Santosh Shukla <santosh.shukla@...aro.org> wrote:
>> > 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.
>> >
>>
>
> So what's wrong with just using the existing read/write API on all
> architectures?
>

nothing wrong, infact read/write api will still be used so to access
mmaped io pci bar at userspace. But right now vfio_pci_map() doesn't
map io pci bar in particular (i.e.. ioresource_io) so I guess need to
add that bar mapping in vfio. pl. correct me if i misunderstood
anything.

>         Arnd
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