Re: [PATCH v2 05/10] ARM: tegra: Rewrite PCIe support as a driver

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On Tue, Jun 12, 2012 at 10:00:26AM -0600, Stephen Warren wrote:
> On 06/12/2012 01:24 AM, Thierry Reding wrote:
> > * Thierry Reding wrote:
> >> AFAICT the even partitioning of the non-prefetchable and 
> >> prefetchable memory regions is arbitrary and it could
> >> potentially be useful to make it configurable via the DT.
> > 
> > So it turns out that this isn't true. But apart from the comments 
> > in the driver I couldn't find any reference to how the prefetch
> > and non-prefetch ranges are partitioned. Judging by the code for
> > Tegra2 this is evenly partitioned among ports 0 and 1 but it would
> > be interesting to know this is done on Tegra3. Is there any way
> > you can find out?
> > 
> > It'd also be nice if some better documentation for the PCIe 
> > controller could be made available. The Tegra2 TRM doesn't contain 
> > any of the AFI or PADS registers and while the Tegra3 TRM
> > documents more registers, their offsets are completely missing.
> 
> I've filed a bug to request better documentation.
> 
> Looking at our downstream driver, the quoted comment below implies
> that the address space layout is pretty arbitrary and programmable at
> least for Tegra30. The code in function
> tegra_pcie_setup_translations() appears to support this (and is
> executed for both Tegra20 and Tegra30, even though the comment for
> Tegra20 doesn't mention any programmability).
> 
> Tegra20:
> 
> > /*
> >  * Tegra2 defines 1GB in the AXI address map for PCIe.
> >  *
> >  * That address space is split into different regions, with sizes and
> >  * offsets as follows:
> >  *
> >  * 0x80000000 - 0x80003fff - PCI controller registers
> >  * 0x80004000 - 0x80103fff - PCI configuration space
> >  * 0x80104000 - 0x80203fff - PCI extended configuration space
> >  * 0x80203fff - 0x803fffff - unused
> >  * 0x80400000 - 0x8040ffff - downstream IO
> >  * 0x80410000 - 0x8fffffff - unused
> >  * 0x90000000 - 0x9fffffff - non-prefetchable memory
> >  * 0xa0000000 - 0xbfffffff - prefetchable memory
> >  */
> 
> (I guess by "is split", it means the driver sets it up that way, not
> that the HW is fixed that way)
> 
> Tegra30:
> 
> > /*
> >  * AXI address map for the PCIe aperture , defines 1GB in the AXI
> >  *  address map for PCIe.
> >  *
> >  *  That address space is split into different regions, with sizes and
> >  *  offsets as follows. Exepct for the Register space, SW is free to slice the
> >  *  regions as it chooces.
> >  *
> >  *  The split below seems to work fine for now.
> >  *
> >  *  0x0000_0000 to 0x00ff_ffff - Register space          16MB.
> >  *  0x0100_0000 to 0x01ff_ffff - Config space            16MB.
> >  *  0x0200_0000 to 0x02ff_ffff - Extended config space   16MB.
> >  *  0x0300_0000 to 0x03ff_ffff - Downstream IO space
> >  *   ... Will be filled with other BARS like MSI/upstream IO etc.
> >  *  0x1000_0000 to 0x1fff_ffff - non-prefetchable memory aperture
> >  *  0x2000_0000 to 0x3fff_ffff - Prefetchable memory aperture
> >  *
> >  *  Config and Extended config sizes are choosen to support
> >  *  maximum of 256 devices,
> >  *  which is good enough for all the current use cases.
> >  *
> >  */
> 
> From
> http://nv-tegra.nvidia.com/gitweb/?p=linux-2.6.git;a=blob;f=arch/arm/mach-tegra/pcie.c;h=b98d4892b5ee46120a9777a969e820a0b5cbb68a;hb=android-tegra-nv-3.1

While it is apparently true that each region can be split arbitrarily this
doesn't seem to be true for the partitioning of each region between the
individual ports.

For instance, while non-prefetchable memory could possibly be mapped either
to 0x90000000 or to 0xa0000000 with an arbitrary size, the subregion that is
assigned to port 0 or 1 seems to be fixed. Judging by the code this currently
seems to be evenly distributed, but it's not so clear how this behaves when
port 1 for example is disabled, or how it is distributed on Tegra30 with all
three ports enabled (split into 3 equally-sized partitions?).

While the upstream driver currently still initializes the I/O and memory
resources for each port (splitting them into equally-sized partitions), the
latest downstream driver no longer seems to do so, but instead just
initializes a global resource for each resource type (tegra_pcie_preinit()).

I hope the improved documentation that you already requested will shed some
light on this.

Thierry

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