On 07/03/2025 at 04:23, David Laight wrote:
> On Thu, 06 Mar 2025 20:29:52 +0900
> Vincent Mailhol via B4 Relay <devnull+mailhol.vincent.wanadoo.fr@xxxxxxxxxx> wrote:
>
>> From: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
>>
>> In an upcoming change, GENMASK() and its friends will indirectly
>> depend on sizeof() which is not available in asm.
>>
>> Instead of adding further complexity to __GENMASK() to make it work
>> for both asm and non asm, just split the definition of the two
>> variants.
> ...
>> +#else /* defined(__ASSEMBLY__) */
>> +
>> +#define GENMASK(h, l) __GENMASK(h, l)
>> +#define GENMASK_ULL(h, l) __GENMASK_ULL(h, l)
>
> What do those actually expand to now?
> As I've said a few times both UL(0) and ULL(0) are just (0) for __ASSEMBLY__
> so the expansions of __GENMASK() and __GENMASK_ULL() contained the
> same numeric constants.
Indeed, in asm, the UL(0) and ULL(0) expands to the same thing: 0.
But the two macros still expand to something different on 32 bits
architectures:
* __GENMASK:
(((~(0)) << (l)) & (~(0) >> (32 - 1 - (h))))
* __GENMASK_ULL:
(((~(0)) << (l)) & (~(0) >> (64 - 1 - (h))))
On 64 bits architecture these are the same.
> This means they should be generating the same values.
> I don't know the correct 'sizeof (int_type)' for the shift right of ~0.
> My suspicion is that a 32bit assembler used 32bit signed integers and a
> 64bit one 64bit signed integers (but a 32bit asm on a 64bit host might
> be 64bit).
> So the asm versions need to avoid the right shift and only do left shifts.
>
> Which probably means they need to be enirely separate from the C versions.
> And then the C ones can have all the ULL() removed.
In this v5, I already have the ULL() removed from the non-uapi C
version. And we are left with two distinct variants:
- the uapi C & asm
- the non-uapi C (including fix width)
For the uapi ones, I do not think we can modify it without a risk of
breaking some random userland. At least, this is not a risk I will take.
And if we have to keep the __GENMASK() and __GENMASK_ULL(), then I would
rather just reuse these for the asm variant instead of splitting further
more and finding ourselves with three variants:
- the uapi C
- the asm
- the non-uapi C (including fix width)
If __GENMASK() and __GENMASK_ULL() were not in the uapi, I would have
agreed with you.
If you believe that the risk of modifying the uapi GENMASK*() is low
enough, then you can submit a patch. But I will definitely not touch
these myself.
Yours sincerely,
Vincent Mailhol
