Re: [PATCH 1/3] iscsi iser: remove DMA restrictions

[Mike Christie <michaelc@cs.wisc.edu>]

James Bottomley wrote:
> On Tue, 2008-02-12 at 15:54 -0500, Pete Wyckoff wrote:
>> iscsi_iser does not have any hardware DMA restrictions.  Add a
>> slave_configure function to remove any DMA alignment restriction,
>> allowing the use of direct IO from arbitrary offsets within a page.
>> Also disable page bouncing; iser has no restrictions on which pages it
>> can address.
>>
>> Signed-off-by: Pete Wyckoff <pw@osc.edu>
>> ---
>>  drivers/infiniband/ulp/iser/iscsi_iser.c |    8 ++++++++
>>  1 files changed, 8 insertions(+), 0 deletions(-)
>>
>> diff --git a/drivers/infiniband/ulp/iser/iscsi_iser.c b/drivers/infiniband/ulp/iser/iscsi_iser.c
>> index be1b9fb..1b272a6 100644
>> --- a/drivers/infiniband/ulp/iser/iscsi_iser.c
>> +++ b/drivers/infiniband/ulp/iser/iscsi_iser.c
>> @@ -543,6 +543,13 @@ iscsi_iser_ep_disconnect(__u64 ep_handle)
>>  	iser_conn_terminate(ib_conn);
>>  }
>>  
>> +static int iscsi_iser_slave_configure(struct scsi_device *sdev)
>> +{
>> +	blk_queue_bounce_limit(sdev->request_queue, BLK_BOUNCE_ANY);
> 
> You really don't want to do this.  That signals to the block layer that
> we have an iommu, although it's practically the same thing as a 64 bit
> DMA mask ... but I'd just leave it to the DMA mask to set this up
> correctly.  Anything else is asking for a subtle bug to turn up years
> from now when something causes the mask and the limit to be mismatched.
> 

I thought BLK_BOUNCE_ANY just meant "don't bounce anything" (that was 
from the blkdev.h comments). We used it for iscsi_tcp because the 
network layer can take any type of page and will do the right thing for 
the hardware it eventually gets sent to.