Re: [PATCH] lib/genaloc: Fix allocation of aligned buffer from non-aligned chunk

[Alexey Skidanov]

On 11/7/18 12:11 AM, Andrew Morton wrote:
> On Tue,  6 Nov 2018 14:20:53 +0200 Alexey Skidanov <alexey.skidanov@xxxxxxxxx> wrote:
> 
>> On success, gen_pool_first_fit_align() returns the bit number such that
>> chunk_start_addr + (bit << order) is properly aligned. On failure,
>> the bitmap size parameter is returned.
>>
>> When the chunk_start_addr isn't aligned properly, the
>> chunk_start_addr + (bit << order) isn't aligned too.
>>
>> To fix this, gen_pool_first_fit_align() takes into account
>> the chunk_start_addr alignment and returns the bit value such that
>> chunk_start_addr + (bit << order) is properly aligned
>> (exactly as it done in CMA).
> 
> Why does this need "fixing"?  Are there current callers which can
> misalign chunk_start_addr?  Or is there a requirement that future
> callers can misalign chunk_start_addr?
> 
I work on adding aligned allocation support for ION heaps
(https://www.spinics.net/lists/linux-driver-devel/msg118754.html). Two
of these heaps use genpool internally.

If you want to have an ability to allocate buffers aligned on different
boundaries (for example, 4K, 1MB, ...), this fix actually removes the
requirement for chunk_start_addr to be aligned on the max possible
alignment.

Thanks,
Alexey