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

[Yong Wu]

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)
> > > 
> > > So in this example, there are no PAs below 0x4000_0000 yet you later
> > > add code to deal with that:
> > > 
> > > > +	/* Workaround for MTK 4GB Mode: Add BIT32 only when PA < 0x4000_0000.*/
> > > > +	if (cfg->oas == ARM_V7S_MTK_4GB_OAS && paddr < 0x40000000UL)
> > > > +		paddr |= BIT_ULL(32);
> > > 
> > > Why? Mainline currently doesn't do anything like this for the "4gb mode"
> > > support as far as I can tell. In fact, we currently unconditionally set
> > > bit 32 in the physical address returned by iova_to_phys() which wouldn't
> > > match your CPU PAs listed above, so I'm confused about how this is supposed
> > > to work.
> > 
> > Actually current mainline have a bug for this. So I tried to use another
> > special patch[1] for it in v8.
> 
> If you're fixing a bug in mainline, I'd prefer to see that as a separate
> patch.
> 
> > But the issue is not critical since MediaTek multimedia consumer(v4l2
> > and drm) don't call iommu_iova_to_phys currently.
> > 
> > > 
> > > 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.

(sorry, My mailbox has something wrong.)