Re: [PATCH v9 08/21] iommu/io-pgtable-arm-v7s: Extend MediaTek 4GB Mode

[Will Deacon]

Ok, I think speaking to Robin helped me a bit with this...

On Thu, Aug 15, 2019 at 06:18:38PM +0800, Yong Wu wrote:
> On Thu, 2019-08-15 at 10:51 +0100, Will Deacon wrote:
> > On Thu, Aug 15, 2019 at 04:47:49PM +0800, Yong Wu wrote:
> > > On Wed, 2019-08-14 at 15:41 +0100, Will Deacon wrote:
> > > > On Sat, Aug 10, 2019 at 03:58:08PM +0800, Yong Wu wrote:
> > > > > MediaTek extend the arm v7s descriptor to support the dram over 4GB.
> > > > > 
> > > > > In the mt2712 and mt8173, it's called "4GB mode", the physical address
> > > > > is from 0x4000_0000 to 0x1_3fff_ffff, but from EMI point of view, it
> > > > > is remapped to high address from 0x1_0000_0000 to 0x1_ffff_ffff, the
> > > > > bit32 is always enabled. thus, in the M4U, we always enable the bit9
> > > > > for all PTEs which means to enable bit32 of physical address. Here is
> > > > > the detailed remap relationship in the "4GB mode":
> > > > > CPU PA         ->    HW PA
> > > > > 0x4000_0000          0x1_4000_0000 (Add bit32)
> > > > > 0x8000_0000          0x1_8000_0000 ...
> > > > > 0xc000_0000          0x1_c000_0000 ...
> > > > > 0x1_0000_0000        0x1_0000_0000 (No change)

[...]

> > > > The way I would like this quirk to work is that the io-pgtable code
> > > > basically sets bit 9 in the pte when bit 32 is set in the physical address,
> > > > and sets bit 4 in the pte when bit 33 is set in the physical address. It
> > > > would then do the opposite when converting a pte to a physical address.
> > > > 
> > > > That way, your driver can call the page table code directly with the high
> > > > addresses and we don't have to do any manual offsetting or range checking
> > > > in the page table code.
> > > 
> > > In this case, the mt8183 can work successfully while the "4gb
> > > mode"(mt8173/mt2712) can not.
> > > 
> > > In the "4gb mode", As the remap relationship above, we should always add
> > > bit32 in pte as we did in [2]. and need add bit32 in the
> > > "iova_to_phys"(Not always add.). That means the "4gb mode" has a special
> > > flow:
> > > a. Always add bit32 in paddr_to_iopte.
> > > b. Add bit32 only when PA < 0x40000000 in iopte_to_paddr.
> > 
> > I think this is probably at the heart of my misunderstanding. What is so
> > special about PAs (is this HW PA or CPU PA?) below 0x40000000? Is this RAM
> > or something else?
> 
> SRAM and HW register that IOMMU can not access.

Ok, so redrawing your table from above, I think we can say something like:


CPU Physical address
====================

0G	1G	2G	3G	4G	5G
|---A---|---B---|---C---|---D---|---E---|
+--I/O--+------------Memory-------------+


IOMMU output physical address
=============================

				4G	5G	6G	7G	8G
				|---E---|---B---|---C---|---D---|
				+------------Memory-------------+


Do you agree? If so, what happens to region 'A' (the I/O region) in the
IOMMU output physical address space. Is it accessible?

Anyway, I think it's the job of the driver to convert between the two
address spaces, so that:

  - On ->map(), bit 32 of the CPU physical address is set before calling
    into the iopgtable code

  - The result from ->iova_to_phys() should be the result from the
    iopgtable code, but with the top bit cleared for addresses over
    5G.

This assumes that:

  1. We're ok setting bit 9 in the ptes mapping region 'E'.
  2. The IOMMU page-table walker uses CPU physical addresses

Are those true?

Thanks,

Will