Re: [PATCH 3/7] ACPI/IORT: Handle memory address size limits as limits

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

On 2023-12-11 1:27 pm, Will Deacon wrote:
> On Wed, Nov 29, 2023 at 05:43:00PM +0000, Robin Murphy wrote:
>> Return the Root Complex/Named Component memory address size limit as an
>> inclusive limit value, rather than an exclusive size.  This saves us
>> having to special-case 64-bit overflow, and simplifies our caller too.
>>
>> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
>> ---
>>   drivers/acpi/arm64/dma.c  |  9 +++------
>>   drivers/acpi/arm64/iort.c | 18 ++++++++----------
>>   include/linux/acpi_iort.h |  4 ++--
>>   3 files changed, 13 insertions(+), 18 deletions(-)
> 
> [...]
> 
>> diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c
>> index 6496ff5a6ba2..eb64d8e17dd1 100644
>> --- a/drivers/acpi/arm64/iort.c
>> +++ b/drivers/acpi/arm64/iort.c
>> @@ -1367,7 +1367,7 @@ int iort_iommu_configure_id(struct device *dev, const u32 *input_id)
>>   { return -ENODEV; }
>>   #endif
>>   
>> -static int nc_dma_get_range(struct device *dev, u64 *size)
>> +static int nc_dma_get_range(struct device *dev, u64 *limit)
>>   {
>>   	struct acpi_iort_node *node;
>>   	struct acpi_iort_named_component *ncomp;
>> @@ -1384,13 +1384,12 @@ static int nc_dma_get_range(struct device *dev, u64 *size)
>>   		return -EINVAL;
>>   	}
>>   
>> -	*size = ncomp->memory_address_limit >= 64 ? U64_MAX :
>> -			1ULL<<ncomp->memory_address_limit;
>> +	*limit = (1ULL << ncomp->memory_address_limit) - 1;
> 
> The old code handled 'ncomp->memory_address_limit >= 64' -- why is it safe
> to drop that? You mention it in the cover letter, so clearly I'm missing
> something!

Because an unsigned shift by 64 or more generates 0 (modulo 2^64), thus 
subtracting 1 results in the correct all-bits-set value for an inclusive 
64-bit limit.

Thanks,
Robin.

>>   
>>   	return 0;
>>   }
>>   
>> -static int rc_dma_get_range(struct device *dev, u64 *size)
>> +static int rc_dma_get_range(struct device *dev, u64 *limit)
>>   {
>>   	struct acpi_iort_node *node;
>>   	struct acpi_iort_root_complex *rc;
>> @@ -1408,8 +1407,7 @@ static int rc_dma_get_range(struct device *dev, u64 *size)
>>   		return -EINVAL;
>>   	}
>>   
>> -	*size = rc->memory_address_limit >= 64 ? U64_MAX :
>> -			1ULL<<rc->memory_address_limit;
>> +	*limit = (1ULL << rc->memory_address_limit) - 1;
> 
> Same thing here.
> 
> Will