Pete Wyckoff wrote:
> [EMAIL PROTECTED] wrote on Mon, 28 Apr 2008 11:20 -0500:
>
>> Erez Zilber wrote:
>>
>>>
>>> I understand that IB's requirements are too difficult to force. I guess
>>> that we will do something like:
>>>
>>> * If the scatterlist is aligned, just use it as is.
>>> * If some elements at the beginning/end of the scatterlist are
>>> unaligned, we can copy only them (and use the rest of the
>>> scatterlist as is). We saw that sometimes (e.g. GFS), only the 1st
>>> element is unaligned, so copying the whole data is unnecessary.
>>> * Else, copy the whole scatterlist to buffers that iSER allocates.
>>> This is what we do today with unaligned buffers.
>>>
>>>
>> Pete, we are discissing the dma_alignment patch you sent. Above are the dma
>> restrictions the iser driver has right now. For the 3rd requirement does
>> that mean we want to set the alignment to be page aligned right?
>>
>
> Erez's alignment rules leave out one case: when the sg list
> has only one element, there are no restrictions on start/end
> alignment.
>
Yeah, this is the trivial case (which is already covered in the iSER code).
> The iser alignment requirements are too complex to express with the
> q->dma_alignment approach. In iser, as Erez suggests, bouncing can
> be done. His second (*) would be a nice addition that would
> definitely help some of our apps.
>
We will try to send a patch for that soon.
> My patch "iscsi iser: remove DMA alignment restriction" changes
> iser to make the alignment 0, rather than the default 512, to avoid
> the bounce buffering in __blk_rq_map_user().
Just to make sure that I understand - "alignment 0 " means that the
block layer doesn't care at all about alignment?
> Else confirming
> single-element or multi-element sg lists will be bounced
> unnecessarily. As bouncing already exists in iser, anything that
> the block layer might do is purely duplicative, so it is best to
> turn it off. This can be done now.
>
> Here it is for reference. I updated the commentary; hopefully
> it makes sense. Expect some fuzz as this is against an older
> 2.6.25-rc4.
>
> -- Pete
>
> commit 47eaf146cde1ddd0a80aa45a1f50514f6a05b928
> Author: Pete Wyckoff <[EMAIL PROTECTED]>
> Date: Fri May 2 10:31:58 2008 -0400
>
> iscsi iser: remove block-layer DMA alignment restriction
>
> iser has rather complex rules for data alignment that come from
> its particular use of memory mapping hardware on IB. iser uses
> its own bouncing layer to satisfy these requirements.
>
> This patch removes the 512-byte DMA alignment requirement that
> is imposed by default at the block layer. Without this change,
> IO from userspace (through sg or bsg, for example) would be
> bounced at the block layer even if that is not required by iser.
>
> Signed-off-by: Pete Wyckoff <[EMAIL PROTECTED]>
>
> diff --git a/drivers/infiniband/ulp/iser/iscsi_iser.c
> b/drivers/infiniband/ulp/iser/iscsi_iser.c
> index be1b9fb..313f102 100644
> --- a/drivers/infiniband/ulp/iser/iscsi_iser.c
> +++ b/drivers/infiniband/ulp/iser/iscsi_iser.c
> @@ -543,6 +543,12 @@ iscsi_iser_ep_disconnect(__u64 ep_handle)
> iser_conn_terminate(ib_conn);
> }
>
> +static int iscsi_iser_slave_configure(struct scsi_device *sdev)
> +{
> + blk_queue_dma_alignment(sdev->request_queue, 0);
> + return 0;
> +}
> +
> static struct scsi_host_template iscsi_iser_sht = {
> .module = THIS_MODULE,
> .name = "iSCSI Initiator over iSER, v." DRV_VER,
> @@ -556,6 +562,7 @@ static struct scsi_host_template iscsi_iser_sht = {
> .eh_device_reset_handler= iscsi_eh_device_reset,
> .eh_host_reset_handler = iscsi_eh_host_reset,
> .use_clustering = DISABLE_CLUSTERING,
> + .slave_configure = iscsi_iser_slave_configure,
> .proc_name = "iscsi_iser",
> .this_id = -1,
> };
>
> >
>
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