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

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Date:   Sat, 26 May 2018 00:33:05 +0530
From:   Jassi Brar <jaswinder.singh@...aro.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Robin Murphy <robin.murphy@....com>,
        Masahisa Kojima <masahisa.kojima@...aro.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>
Subject: Re: [PATCH] net: netsec: reduce DMA mask to 40 bits

On 25 May 2018 at 18:20, Ard Biesheuvel <ard.biesheuvel@...aro.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?

Cheers!