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

[Mike Christie]

Mike Christie wrote:
> Erez Zilber wrote:
>> 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?
> 
> I think what Erez is asking, or maybe it is something I was wondering 
> is, that scsi drivers like lpfc or qla2xxx will do something like:
> 
> if (dma_set_mask(&scsi_host->pdev->dev, DMA_64BIT_MASK))
>     dma_set_mask(&scsi_host->pdev->dev, DMA_32BIT_MASK)
> 
> And when __scsi_alloc_queue calls scsi_calculate_bounce_limit it checks 
> the host's parent dma_mask and sets the bounce_limit for the driver.
> 
> Does srp/iser need to call the dma_set_mask functions or does the 
> ib_device's device already have the dma info set up?

Nevermind. I misread the mail. We know the ib hw driver sets the mask. I 
  guess what we are debating is if we should set the scsi_host's parent 
to the ib_device so the dma mask is picked up, or if should just set 
them in our slave_configure by calling blk_queue_bounce_limit. And if we 
use the blk_queue_bounce_limit path, what function do we call to get the 
dma_mask.

I also modified iser to allocate a host per ib_deivce so it works like 
other scsi drivers since we know the parent. Is this preferred over the 
host per session style. Does it matter?

bnx2i works similar to iser where we use a libiscsi, and dma against a 
real device. Should it do a host per session or host per netdev?

And if we do not allocate a host per ib_device/netdevice what should we 
allocate per those structs? Should we create our own?