Re: [PATCH v4 2/4] s390/pci: store DMA offset in bus_dma_region

[Matthew Rosato <mjrosato@linux.ibm.com>]

>> +static void pci_dma_range_setup(struct pci_dev *pdev)
>> +{
>> +	struct zpci_dev *zdev = to_zpci(pdev);
>> +	struct bus_dma_region *map;
>> +
>> +	map = kzalloc(sizeof(*map), GFP_KERNEL);
>> +	if (!map)
>> +		return;
>> +
>> +	map->cpu_start = 0;
>> +	map->dma_start = PAGE_ALIGN(zdev->start_dma);
>> +	map->size = max(PAGE_ALIGN_DOWN(zdev->end_dma + 1) - map->dma_start, 0);
> 
> Ugh, this is my fault as I suggested it, but this max() doesn't work
> here. The zdev->end_dma is unsigned and so is map->dma_start so if the
> former is smaller underflow will occur and the max() won't save us.
> It's largely a theoretical issue since zdev->end_dma + 1 should always
> be larger than zdev->start_dma, but now the max() looks like we thought
> of that, but then it doesn't work.
> 
> If we handle it maybe just go with:
> 
> aligned_end = PAGE_ALIGN_DOWN(zdev->end_dma + 1);
> if (aligned_end >= map->dma_start)
>      map->size = aligned_end - map->dma_start;
> else
>      map->size = 0;
> 


Given that it's not really something that's supposed to happen, would it make sense then to add a 

WARN_ON_ONCE(map->size == 0);

At the end of this?