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

[David Miller <davem@davemloft.net>]

From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date: Fri, 25 May 2018 14:50:37 +0200

> 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.
> 
> Fixes: 533dd11a12f6 ("net: socionext: Add Synquacer NetSec driver")
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Applied and queued up for -stable.