Re: Marvell MV88SX6041 SATA Driver PIO4 Mode Not DMA

[Jeff Garzik <jgarzik@pobox.com>]

Jens Axboe wrote:
> On Fri, Nov 11 2005, Jens Axboe wrote:
> 
>>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.
> 
> 
> Before commit 31961943e3110c5a1c36b1e0069c29f7c4380e51, sata_mv had
> ->sg_tablesize set to MV_UNDEF which was defined to 0. Just checking,
> 2.6.14 ships with that.

OK, that version has no DMA support, so that makes sense.

I guess the gist of the thread is, sg_tablesize should never be less 
than one, even if "it would hurt DMA performance"?

	Jeff