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

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

On 25/05/18 13:50, Ard Biesheuvel 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.

It's a little bit of a dodge, but until another SoC comes along with 
different requirements I agree it's the reasonable thing to do.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

> Fixes: 533dd11a12f6 ("net: socionext: Add Synquacer NetSec driver")
> Cc: Robin Murphy <robin.murphy@arm.com>
> Cc: Jassi Brar <jaswinder.singh@linaro.org>
> Cc: Masahisa Kojima <masahisa.kojima@linaro.org>
> Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
> Please cc to -stable (v4.16+)
> 
>   drivers/net/ethernet/socionext/netsec.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/socionext/netsec.c b/drivers/net/ethernet/socionext/netsec.c
> index f4c0b02ddad8..59fbf74dcada 100644
> --- a/drivers/net/ethernet/socionext/netsec.c
> +++ b/drivers/net/ethernet/socionext/netsec.c
> @@ -1674,8 +1674,8 @@ static int netsec_probe(struct platform_device *pdev)
>   	if (ret)
>   		goto unreg_napi;
>   
> -	if (dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(64)))
> -		dev_warn(&pdev->dev, "Failed to enable 64-bit DMA\n");
> +	if (dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(40)))
> +		dev_warn(&pdev->dev, "Failed to set DMA mask\n");
>   
>   	ret = register_netdev(ndev);
>   	if (ret) {
>