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From:   Kashyap Desai <kashyap.desai@broadcom.com>
References: <20190605190836.32354-1-hch@lst.de> <20190605190836.32354-11-hch@lst.de>
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Date:   Mon, 17 Jun 2019 14:40:25 +0530
Message-ID: <e7443be50725bbdcdb6f1f4cc73955aa@mail.gmail.com>
Subject: RE: [PATCH 10/13] megaraid_sas: set virt_boundary_mask in the scsi host
To:     Christoph Hellwig <hch@lst.de>
Cc:     Jens Axboe <axboe@kernel.dk>, Sebastian Ott <sebott@linux.ibm.com>,
        Sagi Grimberg <sagi@grimberg.me>,
        Max Gurtovoy <maxg@mellanox.com>,
        Bart Van Assche <bvanassche@acm.org>,
        Ulf Hansson <ulf.hansson@linaro.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        Oliver Neukum <oneukum@suse.com>, linux-block@vger.kernel.org,
        linux-rdma@vger.kernel.org, linux-mmc@vger.kernel.org,
        linux-nvme@lists.infradead.org, linux-scsi@vger.kernel.org,
        "PDL,MEGARAIDLINUX" <megaraidlinux.pdl@broadcom.com>,
        PDL-MPT-FUSIONLINUX <mpt-fusionlinux.pdl@broadcom.com>,
        linux-hyperv@vger.kernel.org, linux-usb@vger.kernel.org,
        usb-storage@lists.one-eyed-alien.net, linux-kernel@vger.kernel.org
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>
> 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.
Thanks for clarification. I have also observed that max_segment_size Is
unchanged and it is 64K.
>
> 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?
Yes. This is a valid configuration.