Pete Wyckoff wrote: > [EMAIL PROTECTED] wrote on Sun, 04 May 2008 11:21 +0300: >> Pete Wyckoff wrote: >>> [EMAIL PROTECTED] wrote on Mon, 28 Apr 2008 11:20 -0500: >>>> Erez Zilber wrote: >>>>> I understand that IB's requirements are too difficult to force. I guess >>>>> that we will do something like: >>>>> >>>>> * If the scatterlist is aligned, just use it as is. >>>>> * If some elements at the beginning/end of the scatterlist are >>>>> unaligned, we can copy only them (and use the rest of the >>>>> scatterlist as is). We saw that sometimes (e.g. GFS), only the 1st >>>>> element is unaligned, so copying the whole data is unnecessary. >>>>> * Else, copy the whole scatterlist to buffers that iSER allocates. >>>>> This is what we do today with unaligned buffers. >>>>> >>>>> >>>> Pete, we are discissing the dma_alignment patch you sent. Above are the >>>> dma >>>> restrictions the iser driver has right now. For the 3rd requirement does >>>> that mean we want to set the alignment to be page aligned right? >>>> >>> Erez's alignment rules leave out one case: when the sg list >>> has only one element, there are no restrictions on start/end >>> alignment. >>> >> Yeah, this is the trivial case (which is already covered in the iSER code). >> >>> The iser alignment requirements are too complex to express with the >>> q->dma_alignment approach. In iser, as Erez suggests, bouncing can >>> be done. His second (*) would be a nice addition that would >>> definitely help some of our apps. >>> >> We will try to send a patch for that soon. >> >>> My patch "iscsi iser: remove DMA alignment restriction" changes >>> iser to make the alignment 0, rather than the default 512, to avoid >>> the bounce buffering in __blk_rq_map_user(). >> Just to make sure that I understand - "alignment 0 " means that the >> block layer doesn't care at all about alignment? > > Yep. The lines to think about are in __blk_rq_map_user: > > alignment = queue_dma_alignment(q) | q->dma_pad_mask; > if (!(uaddr & alignment) && !(len & alignment)) > bio = bio_map_user(q, NULL, uaddr, len, reading); > else > bio = bio_copy_user(q, uaddr, len, reading); > > As it stands, queue_dma_alignment(q) returns 511. That forces the > use of the bio_copy_user path for anything that isn't aligned (start > and end) on a 512 byte boundary. Changing it to page alignment > isn't what we want either. Zero disables the copy path, and lets > iser do everything. You can add printks and test my patch using sg > or bsg to submit from userspace, to see. >
Oh yeah, Pete is right. Since iser has that code already to handle its issues we can avoid all the mess by just setting it to zero. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "open-iscsi" group. To post to this group, send email to open-iscsi@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/open-iscsi -~----------~----~----~----~------~----~------~--~---
