Re: [RFC PATCH 10/17] block: introduce bdev_dax_direct_access()

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On Wed, Feb 1, 2017 at 12:10 AM, Christoph Hellwig <hch@xxxxxx> wrote:
> On Mon, Jan 30, 2017 at 10:16:29AM -0800, Dan Williams wrote:
>> Ok, now that dax_map_atomic() is gone, it's much easier to remove
>> struct blk_dax_ctl.
>>
>> We can also move the partition alignment checks to be a one-time check
>> at bdev_dax_capable() time and kill bdev_dax_direct_access() in favor
>> of calling dax_direct_access() directly.
>
> Yes, please.
>
>> >> +     if ((sector + DIV_ROUND_UP(dax->size, 512))
>> >> +                     > part_nr_sects_read(bdev->bd_part))
>> >> +             return -ERANGE;
>> >> +     sector += get_start_sect(bdev);
>> >> +     return dax_direct_access(dax_inode, sector * 512, &dax->addr,
>> >> +                     &dax->pfn, dax->size);
>> >
>> > And please switch to using bytes as the granularity given that we're
>> > deadling with byte addressable memory.
>>
>> dax_direct_access() does take a byte aligned physical address, but it
>> needs to be at least page aligned since we are returning a pfn_t...
>>
>> Hmm, perhaps the input should be raw page frame number. We could
>> reduce one of the arguments by making the current 'pfn_t *' parameter
>> an in/out-parameter.
>
> In/Out parameters are always a bit problematic in terms of API clarity.
> And updating a device-relative address with an absolute physical one
> sounds like an odd API for sure.

Yes, it does, and I thought better of it shortly after sending that. How about:

long dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
unsigned long nr_pages, void **kaddr, pfn_t *pfn)
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