Messages in this thread | ![/](/images/icornerl.gif) | | Date | Mon, 11 Dec 2023 15:37:55 +0000 | Subject | Re: [PATCH 3/7] ACPI/IORT: Handle memory address size limits as limits | From | Robin Murphy <> |
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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@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.
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