From: Boaz Harrosh <bharrosh@xxxxxxxxxxx>
Subject: Re: [PATCH v2] add bidi support for block pc requests
Date: Wed, 09 May 2007 10:46:34 +0300
> > Roll all the required sglist definitions (request_bufflen,
> > request_buffer, use_sg and sglist_len) into the sgtable pools.
> >
> > We're getting very close to the point where someone gets to sweep
> > through the drivers eliminating the now superfluous non-sg path in the
> > queuecommand. When that happens the only cases become no transfer or SG
> > backed commands. At this point we can do a consolidation of the struct
> > scsi_cmnd fields. This does represent the ideal time to sweep the sg
> > list handling fields into the sgtable and simply have a single pointer
> > to struct sgtable in the scsi_cmnd (== NULL is the signal for a no
> > transfer command).
> >
> This is a grate Idea. Let me see if I understand what you mean.
> 1. An sgtable is a single allocation with an sgtable header type
> at the begining and a veriable size array of struct scatterlist.
> something like:
> struct sgtable {
> struct sgtable_header {
> unsigned sg_count, sglist_len, length;
> struct sgtable* next; //for Jens's big io
> } hdr;
> struct scatterlist sglist[];
> }
Can we have more simple sgtable?
struct sgtable {
unsigned use_sg;
unsigned length;
unsigned sglist_len;
struct scatterlist sglist[0];
};
Then we could do something like this:
struct scsi_host_sgtable_pool {
size_t size;
char *name;
struct kmem_cache *slab;
mempool_t *pool;
};
int __init scsi_init_queue(void)
{
for (i = 0; i < SG_MEMPOOL_NR; i++) {
struct scsi_host_sgtable_pool *sgtbp = scsi_sgtable_pools + i;
int size = sizeof(struct sgtable) + sgp->size * sizeof(struct scatterlist);
sgp->slab = kmem_cache_create(sgp->name, size, 0,
SLAB_HWCACHE_ALIGN, NULL, NULL);
if (!sgp->slab) {
printk(KERN_ERR "SCSI: can't init sg slab %s\n",
sgp->name);
}
sgp->pool = mempool_create_slab_pool(SG_MEMPOOL_SIZE,
sgp->slab);
Jens' chaining sg lists adds sg->next so we don't need
sgtable->next. We can just add __use_sg to struct sgtable.
> 2. The way we can do this in stages: Meaning put up code that has
> both sets of API, Transfer drivers one-by-one to new API, deprecate
> old API for a kernel cycle or two. Than submit last piece of
> code that removes the old API.
> It can be done. We just need to copy sgtable_header fields
> to the old fields, and let them stick around for a while.
That's not bad, but can we convert all the LLDs all at once?
The changes to scsi mid-layer are almost done. Probabaly, you can just
add sgtable stuff (and scsi_dma_map/scsi_dma_unmap helper functions
that Christoph proposed long ago) to my previous patch. It can be done
for several hours. So you have enough time for the LLDs' changes.
> 3. The second bidi sgtable will hang on request->next_rq->special.
I think so.
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