Hi, Liviu,


On 11/11/2016 10:45 PM, liviu.dudau@arm.com wrote:

On Fri, Nov 11, 2016 at 01:39:35PM +0000, Gabriele Paoloni wrote:

quoted

Hi Arnd

quoted

-----Original Message-----
From: Arnd Bergmann [mailto:arnd@arndb.de]
Sent: 10 November 2016 16:07
To: Gabriele Paoloni
Cc: linux-arm-kernel@lists.infradead.org; Yuanzhichang;
mark.rutland@arm.com; devicetree@vger.kernel.org;
lorenzo.pieralisi@arm.com; minyard@acm.org; linux-pci@vger.kernel.org;
benh@kernel.crashing.org; John Garry; will.deacon@arm.com; linux-
kernel@vger.kernel.org; xuwei (O); Linuxarm; zourongrong@gmail.com;
robh+dt@kernel.org; kantyzc@163.com; linux-serial@vger.kernel.org;
catalin.marinas@arm.com; olof@lixom.net; liviu.dudau@arm.com;
bhelgaas@googl e.com; zhichang.yuan02@gmail.com
Subject: Re: [PATCH V5 3/3] ARM64 LPC: LPC driver implementation on
Hip06

On Thursday, November 10, 2016 3:36:49 PM CET Gabriele Paoloni wrote:

quoted

Where should we get the range from? For LPC we know that it is going
Work on anything that is not used by PCI I/O space, and this is
why we use [0, PCIBIOS_MIN_IO]

It should be allocated the same way we allocate PCI config space
segments. This is currently done with the io_range list in
drivers/pci/pci.c, which isn't perfect but could be extended
if necessary. Based on what others commented here, I'd rather
make the differences between ISA/LPC and PCI I/O ranges smaller
than larger.

Gabriele,

quoted

I am not sure this would make sense...

IMHO all the mechanism around io_range_list is needed to provide the
"mapping" between I/O tokens and physical CPU addresses.

Currently the available tokens range from 0 to IO_SPACE_LIMIT.

As you know the I/O memory accessors operate on whatever
__of_address_to_resource sets into the resource (start, end).

With this special device in place we cannot know if a resource is
assigned with an I/O token or a physical address, unless we forbid
the I/O tokens to be in a specific range.

So this is why we are changing the offsets of all the functions
handling io_range_list (to make sure that a range is forbidden to
the tokens and is available to the physical addresses).

We have chosen this forbidden range to be [0, PCIBIOS_MIN_IO)
because this is the maximum physical I/O range that a non PCI device
can operate on and because we believe this does not impose much
restriction on the available I/O token range; that now is 
[PCIBIOS_MIN_IO, IO_SPACE_LIMIT].
So we believe that the chosen forbidden range can accommodate
any special ISA bus device with no much constraint on the rest
of I/O tokens...

Your idea is a good one, however you are abusing PCIBIOS_MIN_IO and you
actually need another variable for "reserving" an area in the I/O space
that can be used for physical addresses rather than I/O tokens.

I think selecting PCIBIOS_MIN_IO as the separator of mapped and non-mapped I/O
range probably is not so reasonable.
PCIBIOS_MIN_IN is specific to PCI devices, it seems as the recommended minimal
start I/O address when assigning the pci device I/O region. It is probably not
defined in some platforms/architectures when no PCI is needed there. That is why
my patch caused some compile error on some archs;

But more important thing is that the PCIBIOS_MIN_IO has different value on
different platforms/architectures. On Arm64, it is 4K currently, but in other
archs, it is not true. And the maximum LPC I/O address should be 64K
theoretically, although for compatible ISA, 2K is enough.
So, It means using PCIBIOS_MIN_IO on arm64 can match our I/O reservation
require. But we can not make this indirectIO work well on other architectures.

I am thinking Arnd's suggestion. But I worry about I haven't completely
understood his idea. What about create a new bus host for LPC/ISA whose I/O
range can be 64KB? This LPC/ISA I/O range works similar to PCI host bridge's I/O
window, all the downstream devices under LPC/ISA should request I/O from that
root resource. But it seems Arnd want this root resource registered dynamically,
I am not sure how to do...

Anyway, if we have this root I/O resource, we don't need any new macro or
variable for the LPC/ISA I/O reservation.

Hope my thought is right.

Best,
Zhichang

The one good example for using PCIBIOS_MIN_IO is when your platform/architecture
does not support legacy ISA operations *at all*. In that case someone
sets the PCIBIOS_MIN_IO to a non-zero value to reserve that I/O range
so that it doesn't get used. With Zhichang's patch you now start forcing
those platforms to have a valid address below PCIBIOS_MIN_IO.

For the general case you also have to bear in mind that PCIBIOS_MIN_IO could
be zero. In that case, what is your "forbidden" range? [0, 0) ? So it makes
sense to add a new #define that should only be defined by those architectures/
platforms that want to reserve on top of PCIBIOS_MIN_IO another region
where I/O tokens can't be generated for.

Best regards,
Liviu

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Your current version has

        if (arm64_extio_ops->pfout)                             \
                arm64_extio_ops->pfout(arm64_extio_ops->devpara,\
                       addr, value, sizeof(type));             \

Instead, just subtract the start of the range from the logical
port number to transform it back into a bus-local port number:

These accessors do not operate on IO tokens:

If (arm64_extio_ops->start > addr || arm64_extio_ops->end < addr)
addr is not going to be an I/O token; in fact patch 2/3 imposes that
the I/O tokens will start at PCIBIOS_MIN_IO. So from 0 to

PCIBIOS_MIN_IO

quoted

we have free physical addresses that the accessors can operate on.

Ah, I missed that part. I'd rather not use PCIBIOS_MIN_IO to refer to
the logical I/O tokens, the purpose of that macro is really meant
for allocating PCI I/O port numbers within the address space of
one bus.

As I mentioned above, special devices operate on CPU addresses directly,
not I/O tokens. For them there is no way to distinguish....

quoted

Note that it's equally likely that whichever next platform needs
non-mapped I/O access like this actually needs them for PCI I/O space,
and that will use it on addresses registered to a PCI host bridge.

Ok so here you are talking about a platform that has got an I/O range
under the PCI host controller, right?
And this I/O range cannot be directly memory mapped but needs special
redirections for the I/O tokens, right?

In this scenario registering the I/O ranges with the forbidden range
implemented by the current patch would still allow to redirect I/O
tokens as long as arm64_extio_ops->start >= PCIBIOS_MIN_IO

So effectively the special PCI host controller
1) knows the physical range that needs special redirection
2) register such range
3) uses pci_pio_to_address() to retrieve the IO tokens for the
   special accessors
4) sets arm64_extio_ops->start/end to the IO tokens retrieved in 3)

So to be honest I think this patch can fit well both with
special PCI controllers that need I/O tokens redirection and with
special non-PCI controllers that need non-PCI I/O physical
address redirection...

Thanks (and sorry for the long reply but I didn't know how
to make the explanation shorter :) )

Gab

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If we separate the two steps:

a) assign a range of logical I/O port numbers to a bus
b) register a set of helpers for redirecting logical I/O
   port to a helper function

then I think the code will get cleaner and more flexible.
It should actually then be able to replace the powerpc
specific implementation.

	Arnd