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

[Robin Murphy]

On 2023-12-11 3:30 pm, Will Deacon wrote:
> On Mon, Dec 11, 2023 at 03:01:27PM +0000, Robin Murphy wrote:
>> 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 at 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.
> 
> Oh, I'd have thought you'd have gotten one of those "left shift count >=
> width of type" warnings if you did that.

Compilers might give such a warning if it was a constant shift whose 
size was visible at compile time, but even then only because compilers 
seem to have a vendetta against us relying on the well-defined 
behaviours of unsigned integer overflow (it's only *signed* shifts which 
are UB if the result is unrepresentable).

Cheers,
Robin.