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

On Tue, 2015-12-29 at 22:00 +0530, Santosh Shukla wrote:

On Tue, Dec 29, 2015 at 9:50 PM, Arnd Bergmann [off-list ref] wrote:

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On Tuesday 29 December 2015 21:25:15 Santosh Shukla wrote:

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mistakenly added wrong email-id of alex, looping his correct one.

On 29 December 2015 at 21:23, Santosh Shukla <santosh.shukla@lina
ro.org> wrote:

quoted

On 29 December 2015 at 18:58, Arnd Bergmann [off-list ref]
wrote:

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On Wednesday 23 December 2015 17:04:40 Santosh Shukla wrote:

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On 23 December 2015 at 03:26, Arnd Bergmann [off-list ref]
wrote:

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On Tuesday 22 December 2015, Santosh Shukla wrote:

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}

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

vfio_pci_mmap(), the read/write accessors fully support i/o port.

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.

Maybe I misunderstood what you were asking for, it seemed like you
specifically wanted to be able to mmap i/o port space, which is
possible, just not something we can do on x86.  Maybe I should have
asked why.  The vfio API already supports read/write access to i/o port
space, so if you intend to mmap it only to use read/write on top of the
mmap, I suppose you might see some performance improvement, but not
really any new functionality.  You'd also need to deal with page size
issues since i/o port ranges are generally quite a bit smaller than the
host page size and they'd need to be mapped such that each devices does
not share a host page of i/o port space with other devices.  On x86 i/o
port space is mostly considered legacy and not a performance critical
path for most modern devices; PCI SR-IOV specifically excludes i/o port
space.  So what performance gains do you expect to see in being able to
mmap i/o port space and what hardware are you dealing with that relies
on i/o port space rather than mmio for performance?  Thanks,

Alex