Re: Marvell MV88SX6041 SATA Driver PIO4 Mode Not DMA

[Jens Axboe <axboe@suse.de>]

On Fri, Nov 11 2005, Jeff Garzik wrote:
> Jens Axboe wrote:
> >On Fri, Nov 11 2005, Jeff Garzik wrote:
> >
> >>Spencer Tuttle wrote:
> >>
> >>>I have just compiled the new 2.6.14 kernel from the gentoo-sources tree.
> >>>
> >>>I can access the drives just fine, but it seems really slow.  Here is
> >>>the dmesg output when I load the kernel module
> >>
> >>That's expected, since the driver in 2.6.14 only does PIO mode.
> >>
> >>Try 2.6.14-gitN which supports EDMA.
> >
> >
> >Did you see these as well:
> >
> >blk_queue_max_hw_segments: set to minimum 1
> >
> >Could it be forgetting to set ->sg_tablesize as well?
> 
> [jgarzik@sata linux-2.6]$ grep MV_MAX_SG_CT drivers/scsi/sata_mv.c
>         MV_MAX_SG_CT            = 176,
>         MV_SG_TBL_SZ            = (16 * MV_MAX_SG_CT),
>         .sg_tablesize           = MV_MAX_SG_CT,

It was just a guess, clearly someone is calling scsi_alloc_queue() with
an sdev->host->sg_tablesize of 0. While that wont make PIO a whole lot
faster, it's clearly a bug and would certainly hurt DMA performance.

-- 
Jens Axboe