Re: [PATCH v3 1/4] libata: add R-Car SATA driver

[Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>]

Hello.

On 05/26/2013 04:23 AM, Tejun Heo wrote:

>
>>     Because with SFF-8038i  (BMIDE) controllers transfer can't cross
>> 64KiB address boundaries. This function has to break up those S/G
>> entries that do cross them. I.e. the PRD length is actually additionally
>> limited by an offset of its start within 64KiB memory segment.
>> Does this make sense? Or does the block layer already care about this?
>> Hm, I'm not confident now, should revisit this after some sleep.
> Yeah, that's exactly where I'm confused.  ATA_DMA_BOUNDARY is 64k
> which becomes both queue_segment_boundary and dma seg_boundary.
> AFAICS, both __blk_segment_map_sg() and dma mapping won't merge across
> seg_boundary and as each bvec is a single page at most, we shouldn't
> need to worry about getting sg's which cross 64k boundaries in bmdma
> controllers.  Hmmmmm...... I gotta be missing something.  What am I
> missing here?

     Probably nothing... maybe Jeff just copied that from ATADRVR
(which was his inspiration IIRC).
    BTW, I've just done some experimentation with my R-Car target
and ATA_DEBUG/ATA_VERBOSE_DEBUG #define'd and it turned out
that block layer didn't merge any segments at all... :-/

> Thanks.
>

WBR, Sergei