Re: [PATCH] ACPI/IORT: Do not blindly trust DMA masks from firmware

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

On 2021-01-21 19:16, Moritz Fischer wrote:
> Address issue observed on real world system with suboptimal IORT table
> where DMA masks of PCI devices would get set to 0 as result.
> 
> iort_dma_setup() would query the root complex' IORT entry for a DMA
> mask, and use that over the one the device has been configured with
> earlier.
> 
> Ideally we want to use the minimum mask of what the IORT contains for
> the root complex and what the device was configured with, but never 0.
> 
> Fixes: 5ac65e8c8941 ("ACPI/IORT: Support address size limit for root complexes")
> Signed-off-by: Moritz Fischer <mdf@kernel.org>
> ---
> Hi all,
> 
> not sure I'm doing this right, but I think the current behavior (while a
> corner case) seems to also fail for 32 bit devices if the IORT specifies
> 64 bit. It works on my test system now with a 32 bit device.

I suppose it could go wrong if it's an old driver that doesn't 
explicitly set its own masks and assumes they will always be 32-bit. 
Technically we'd consider that the driver's fault these days, but 
there's a lot of legacy around still.

> Open to suggestions for better solutions (and maybe the
> nc_dma_get_range() should have the same sanity check?)

Honestly the more I come back to this, the more I think we should give 
up trying to be clever and just leave the default masks alone beyond the 
initial "is anything set up at all?" sanity checks. Setting the bus 
limit is what really matters these days, and should be sufficient to 
encode any genuine restriction. There's certainly no real need to widen 
the default masks above 32 bits just because firmware suggests so, since 
the driver should definitely be calling dma_set_mask() and friends later 
if it's >32-bit capable anyway.

> Thanks,
> Moritz
> 
> ---
>   drivers/acpi/arm64/iort.c | 11 ++++++++---
>   1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c
> index d4eac6d7e9fb..c48eabf8c121 100644
> --- a/drivers/acpi/arm64/iort.c
> +++ b/drivers/acpi/arm64/iort.c
> @@ -1126,6 +1126,11 @@ static int rc_dma_get_range(struct device *dev, u64 *size)
>   
>   	rc = (struct acpi_iort_root_complex *)node->node_data;
>   
> +	if (!rc->memory_address_limit) {
> +		dev_warn(dev, "Root complex has broken memory_address_limit\n");

Probably warrants a FW_BUG in there.

> +		return -EINVAL;
> +	}
> +
>   	*size = rc->memory_address_limit >= 64 ? U64_MAX :
>   			1ULL<<rc->memory_address_limit;
>   
> @@ -1172,9 +1177,9 @@ void iort_dma_setup(struct device *dev, u64 *dma_addr, u64 *dma_size)
>   		 */
>   		end = dmaaddr + size - 1;
>   		mask = DMA_BIT_MASK(ilog2(end) + 1);
> -		dev->bus_dma_limit = end;
> -		dev->coherent_dma_mask = mask;
> -		*dev->dma_mask = mask;
> +		dev->bus_dma_limit = min_not_zero(dev->bus_dma_limit, end);

This doesn't need to change, since the default bus limit is 0 anyway 
(and that means "no limit").

> +		dev->coherent_dma_mask = min_not_zero(dev->coherent_dma_mask, mask);
> +		*dev->dma_mask = min_not_zero(*dev->dma_mask, mask);

AFAICS the only way an empty mask could get here now is from 
nc_dma_get_range(), so I'd rather see a consistent warning there than 
just silently start working around that too.

Of course IORT doesn't say these fields are optional (other than the 
lack of a root complex limit in older table versions), so we're giving 
bad firmware a pass to never be fixed, ho hum...

Thanks,
Robin.

>   	}
>   
>   	*dma_addr = dmaaddr;
>