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

[Gabriele Paoloni <gabriele.paoloni@huawei.com>]

Hi Liviu

> -----Original Message-----
> From: liviu.dudau@arm.com [mailto:liviu.dudau@arm.com]
> Sent: 11 November 2016 14:46
> To: Gabriele Paoloni
> Cc: Arnd Bergmann; 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; bhelgaas@googl e.com;
> zhichang.yuan02@gmail.com
> Subject: Re: [PATCH V5 3/3] ARM64 LPC: LPC driver implementation on
> Hip06
> 
> 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.

But if PCIBIOS_MIN_IO is 0 then it means that all I/O space is to be used
by PCI controllers only...so if you have a special bus device using
an I/O range in this case should be a PCI controller...i.e. I would
expect it to fall back into the case of I/O tokens redirection rather than
physical addresses redirection (as mentioned below from my previous reply).
What do you think?

Thanks

Gab


> 
> 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
> 
> --
> ====================
> | I would like to |
> | fix the world,  |
> | but they're not |
> | giving me the   |
>  \ source code!  /
>   ---------------
>     ¯\_(ツ)_/¯