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

[Erez Zilber]

Erez Zilber wrote:
>
> Pete Wyckoff wrote:
> > James.Bottomley at HansenPartnership.com wrote on Tue, 12 Feb 2008
> 15:57 -0600:
> >  
> >> On Tue, 2008-02-12 at 16:46 -0500, Pete Wyckoff wrote:
> >>    
> >>> James.Bottomley at HansenPartnership.com wrote on Tue, 12 Feb 2008
> 15:10 -0600:
> >>>      
> >>>> 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 at 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.
> >>>>        
> >>> Oh.  I decided to add that line for symmetry with TCP, and was
> >>> convinced by the arguments here:
> >>>
> >>>     commit b6d44fe9582b9d90a0b16f508ac08a90d899bf56
> >>>     Author: Mike Christie <michaelc at cs.wisc.edu>
> >>>     Date:   Thu Jul 26 12:46:47 2007 -0500
> >>>
> >>>     [SCSI] iscsi_tcp: Turn off bounce buffers
> >>>
> >>>     It was found by LSI that on setups with large amounts of memory
> >>>     we were bouncing buffers when we did not need to. If the iscsi tcp
> >>>     code touches the data buffer (or a helper does),
> >>>     it will kmap the buffer. iscsi_tcp also does not interact with
> hardware,
> >>>     so it does not have any hw dma restrictions. This patch sets
> the bounce
> >>>     buffer settings for our device queue so buffers should not be
> bounced
> >>>     because of a driver limit.
> >>>
> >>> I don't see a convenient place to callback into particular iscsi
> >>> devices to set the DMA mask per-host.  It has to go on the
> >>> shost_gendev, right?, but only for TCP and iSER, not qla4xxx, which
> >>> handles its DMA mask during device probe.
> >>>      
> >> You should be taking your mask from the underlying infiniband device as
> >> part of the setup, shouldn't you?
> >>    
> >
> > I think you're right about this.  All the existing IB HW tries to
> > set a 64-bit dma mask, but that's no reason to disable the mechanism
> > entirely in iser.  I'll remove that line that disables bouncing in
> > my patch.  Perhaps Mike will know if the iscsi_tcp usage is still
> > appropriate.
> >
> >  
>
> Let me make sure that I understand: you say that the IB HW driver (e.g.
> ib_mthca) tries to set a 64-bit dma mask:
>
>     err = pci_set_dma_mask(pdev, DMA_64BIT_MASK);
>     if (err) {
>         dev_warn(&pdev->dev, "Warning: couldn't set 64-bit PCI DMA
> mask.\n");
>         err = pci_set_dma_mask(pdev, DMA_32BIT_MASK);
>         if (err) {
>             dev_err(&pdev->dev, "Can't set PCI DMA mask, aborting.\n");
>             goto err_free_res;
>         }
>     }
>
> So, in the example above, the driver will use a 64-bit mask or a 32-bit
> mask (or fail). According to that, iSER (and SRP) needs to call
> blk_queue_bounce_limit with the appropriate parameter, right?
>

Roland, James,

I'm trying to fix this potential problem in iSER, and I have some
questions about that. How can I get the DMA mask that the HCA driver is
using (DMA_64BIT_MASK or DMA_32BIT_MASK)? Can I get it somehow from
struct ib_device? Is it in ib_device->device?

Another question is - after I get the DMA mask data from the HCA driver,
I guess that I need to call blk_queue_bounce_limit with the appropriate
parameter (BLK_BOUNCE_HIGH, BLK_BOUNCE_ANY or BLK_BOUNCE_ISA). Which
value should iSER use according to the DMA mask info? For example, if
the HCA driver sets DMA_64BIT_MASK, should iSER use
BLK_BOUNCE_HIGH/BLK_BOUNCE_ANY/BLK_BOUNCE_ISA ?

Thanks,
Erez