Re: [PATCH] net: netsec: reduce DMA mask to 40 bits

[Robin Murphy <robin.murphy@arm.com>]

On Sat, 26 May 2018 00:33:05 +0530
Jassi Brar <jaswinder.singh@linaro.org> wrote:

> On 25 May 2018 at 18:20, Ard Biesheuvel <ard.biesheuvel@linaro.org>
> wrote:
> > The netsec network controller IP can drive 64 address bits for DMA,
> > and the DMA mask is set accordingly in the driver. However, the
> > SynQuacer SoC, which is the only silicon incorporating this IP at
> > the moment, integrates this IP in a manner that leaves address bits
> > [63:40] unconnected.
> >
> > Up until now, this has not resulted in any problems, given that the
> > DDR controller doesn't decode those bits to begin with. However,
> > recent firmware updates for platforms incorporating this SoC allow
> > the IOMMU to be enabled, which does decode address bits [47:40],
> > and allocates top down from the IOVA space, producing DMA addresses
> > that have bits set that have been left unconnected.
> >
> > Both the DT and ACPI (IORT) descriptions of the platform take this
> > into account, and only describe a DMA address space of 40 bits
> > (using either dma-ranges DT properties, or DMA address limits in
> > IORT named component nodes). However, even though our IOMMU and bus
> > layers may take such limitations into account by setting a narrower
> > DMA mask when creating the platform device, the netsec probe()
> > entrypoint follows the common practice of setting the DMA mask
> > uncondionally, according to the capabilities of the IP block itself
> > rather than to its integration into the chip.
> >
> > It is currently unclear what the correct fix is here. We could hack
> > around it by only setting the DMA mask if it deviates from its
> > default value of DMA_BIT_MASK(32). However, this makes it
> > impossible for the bus layer to use DMA_BIT_MASK(32) as the bus
> > limit, and so it appears that a more comprehensive approach is
> > required to take DMA limits imposed by the SoC as a whole into
> > account.
> >
> > In the mean time, let's limit the DMA mask to 40 bits. Given that
> > there is currently only one SoC that incorporates this IP, this is
> > a reasonable approach that can be backported to -stable and buys us
> > some time to come up with a proper fix going forward.
> >  
> I am sure you already thought about it, but why not let the platform
> specify the bit mask for the driver (via some "bus-width" property),
> to override the default 64 bit mask?

Because lack of a property to describe the integration is not the
problem. There are already at least two ways: the general DT/IORT
properties for describing DMA addressing - which it would be a bit
ungainly for a driver to parse for this reason, but not impossible -
and inferring it from a SoC-specific compatible - which is more
appropriate, and what we happen to be able to do here.

Robin.