[3427] in linux-scsi channel archive
Idea for SCSI kernel drivers: merge buffers
daemon@ATHENA.MIT.EDU (Marc SCHAEFER)
Sun Mar 1 13:16:42 1998
From: Marc SCHAEFER <schaefer@alphanet.ch>
Date: 1 Mar 1998 19:00:40 +0100
Apparently-To: linux-scsi@vger.rutgers.edu
To: ;@unlisted-recipients (no To-header on input)
Most SCSI drivers (low-level) use scatter/gather. But do they
do something like this to not use too much memory in the
scripts of the hardware for mailboxes or scatter/gather
entries ?
if (Cmnd->use_sg) {
sg = (struct scatterlist *) Cmnd->request_buffer;
ds = cmd->dataseg;
#if 0 /* standard */
cmd->segment_cnt = Cmnd->use_sg;
for (sg_count = 0; sg_count < Cmnd->use_sg; sg_count++) {
ds->d_base = (void *) virt_to_bus(sg->address);
ds->d_count = sg->length;
sg++;
ds++;
}
#else /* trying to merge blocks fast */
/* Initialisation */
ds->d_base = (void *) virt_to_bus(sg->address);
ds->d_count = sg->length;
sg++;
sg_count = 1;
block_count = 1;
while (sg_count < Cmnd->use_sg) {
void *d_base = (void *) virt_to_bus(sg->address);
if ((ds->d_base + ds->d_count) == (d_base)) {
ds->d_count += sg->length; /* BUG: no overlap check */
DEBUG_INTR(printk("merged, cnt=%d size=%ld\n", block_count, ds->d_count);)
}
else {
/* We can't reuse the old one, commit it */
ds++;
block_count++;
ds->d_base = (void *) virt_to_bus(sg->address);
ds->d_count = sg->length;
}
sg++;
sg_count++;
}
cmd->segment_cnt = block_count;
#endif
}
The idea is to merge mailboxes/sg slots if the physical (DMA) address
are contiguous. Of course, integrity checks on the size of the
mailboxes and stuff has to be done before.
I saw that it happens all the times with the fs (at least in 2.0.x), and
it is bad to eat slots for nothing.
PS: I found that because the driver I am writing, for the Fujitsu MB86606,
and I have not yet used the ``user program'' (ie the SCRIPTS engine),
and thus I do scatter/gather with the CPU. This small optimization
looked like it improved a bit my performance. The above is GPL'd
from now on :-) Maybe already someone had this idea. If you
find a bug in the above, report it. Looks like it works. Next
step is writing the user program :-)
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