linux-kernel - Re: [PATCH 2/2] virtio/virtio_ring: Fix the dma_max_mapping

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Date:   Thu, 25 Jul 2019 15:04:16 +0200
From:   Christoph Hellwig <hch@....de>
To:     Auger Eric <eric.auger@...hat.com>
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>, eric.auger.pro@...il.com,
        m.szyprowski@...sung.com, mst@...hat.com, jasowang@...hat.com,
        virtualization@...ts.linux-foundation.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] virtio/virtio_ring: Fix the dma_max_mapping_size
 call

On Thu, Jul 25, 2019 at 01:53:49PM +0200, Auger Eric wrote:
> I am confused: if vring_use_dma_api() returns false if the dma_mask is
> unset (ie. vring_use_dma_api() returns false), the virtio-blk-pci device
> will not be able to get translated addresses and won't work properly.
> 
> The patch above allows the dma api to be used and only influences the
> max_segment_size and it works properly.
> 
> So is it normal the dma_mask is unset in my case?

Its not normal.  I assume you use virtio-nmio?  Due to the mess with
the dma_mask being a pointer a lot of subsystems forgot to set a dma
mask up, and oddly enough seem to mostly get away with it.