Subject: Re: [PATCH 00/11] of: dma-ranges fixes and improvements

From: Thierry Reding <[email-masked]>
Date: Mon, 30 Sep 2019 10:56:06 +0200

On Mon, Sep 30, 2019 at 01:20:55AM -0700, Christoph Hellwig wrote:
> On Sun, Sep 29, 2019 at 01:16:20PM +0200, Arnd Bergmann wrote:
> > On a semi-related note, Thierry asked about one aspect of the dma-ranges
> > property recently, which is the behavior of dma_set_mask() and related
> > functions when a driver sets a mask that is larger than the memory
> > area in the bus-ranges but smaller than the available physical RAM.
> > As I understood Thierry's problem and the current code, the generic
> > dma_set_mask() will either reject the new mask entirely or override
> > the mask set by of_dma_configure, but it fails to set a correct mask
> > within the limitations of the parent bus in this case.
> 
> There days dma_set_mask will only reject a mask if it is too small
> to be supported by the hardware.  Larger than required masks are now
> always accepted.

Summarizing why this came up: the memory subsystem on Tegra194 has a
mechanism controlled by bit 39 of physical addresses. This is used to
support two variants of sector ordering for block linear formats. The
GPU uses a slightly different ordering than other MSS clients, so the
drivers have to set this bit depending on who they interoperate with.

I was running into this as I was adding support for IOMMU support for
the Ethernet controller on Tegra194. The controller has a HW feature
register that contains how many address bits it supports. This is 40
for Tegra194, corresponding to the number of address bits to the MSS.
Without IOMMU support that's not a problem because there are no systems
with 40 bits of system memory. However, if we enable IOMMU support, the
DMA/IOMMU code will allocate from the top of a 48-bit (constrained to
40 bits via the DMA mask) input address space. This causes bit 39 to be
set, which in turn will make the MSS reorder sectors and break network
communications.

Since this reordering takes place at the MSS level, this applies to all
MSS clients. Most of these clients always want bit 39 to be 0, whereas
the clients that can and want to make use of the reordering always want
bit 39 to be under their control, so they can control in a fine-grained
way when to set it.

This means that effectively all MSS clients can only address 39 bits, so
instead of hard-coding that for each driver I thought it'd make sense to
have a central place to configure this, so that all devices by default
are restricted to 39-bit addressing. However, with the current DMA API
implementation this causes a problem because the default 39-bit DMA mask
would get overwritten by the driver (as in the example of the Ethernet
controller setting a 40-bit DMA mask because that's what the hardware
supports).

I realize that this is somewhat exotic. On one hand it is correct for a
driver to say that the hardware supports 40-bit addressing (i.e. the
Ethernet controller can address bit 39), but from a system integration
point of view, using bit 39 is wrong, except in a very restricted set of
cases.

If I understand correctly, describing this with a dma-ranges property is
the right thing to do, but it wouldn't work with the current
implementation because drivers can still override a lower DMA mask with
a higher one.

Thierry
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