Re: [PATCH 10/13] megaraid_sas: set virt_boundary_mask in the scsi host

From: Christoph Hellwig
Date: Mon Jun 17 2019 - 04:49:59 EST

On Fri, Jun 14, 2019 at 01:28:47AM +0530, Kashyap Desai wrote:
> Is there any changes in API blk_queue_virt_boundary? I could not find
> relevant code which account for this. Can you help ?
> Which git repo shall I use for testing ? That way I can confirm, I didn't
> miss relevant changes.

Latest mainline plus the series (which is about to get resent).
blk_queue_virt_boundary now forced an unlimited max_hw_sectors as that
is how PRP-like schemes work, to work around a block driver merging
bug. But we also need to communicate that limit to the DMA layer so
that we don't set a smaller iommu segment size limitation.

> >From your above explanation, it means (after this patch) max segment size
> of the MR controller will be set to 4K.
> Earlier it is possible to receive single SGE of 64K datalength (Since max
> seg size was 64K), but now the same buffer will reach the driver having 16
> SGEs (Each SGE will contain 4K length).

No, there is no more limit for the size of the segment at all,
as for PRPs each PRP is sort of a segment from the hardware perspective.
We just require the segments to not have gaps, as PRPs don't allow for

That being said I think these patches are wrong for the case of megaraid
or mpt having both NVMe and SAS/ATA devices behind a single controller.
Is that a valid configuration?