Re: [PATCH V5 3/3] ARM64 LPC: LPC driver implementation on Hip06

["liviu.dudau@arm.com" <liviu.dudau@arm.com>]

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

> 
> 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.

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

> 
> > 
> > > > 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
> > > 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....
> 
> > 
> > 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
> 
> > 
> > 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

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