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Date:   Tue, 9 Aug 2022 20:54:03 +0200
From:   "hch@lst.de" <hch@lst.de>
To:     Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc:     "Hellstrom, Thomas" <thomas.hellstrom@intel.com>,
        "daniel@ffwll.ch" <daniel@ffwll.ch>,
        "joonas.lahtinen@linux.intel.com" <joonas.lahtinen@linux.intel.com>,
        "Vivi, Rodrigo" <rodrigo.vivi@intel.com>,
        "bob.beckett@collabora.com" <bob.beckett@collabora.com>,
        "jani.nikula@linux.intel.com" <jani.nikula@linux.intel.com>,
        "airlied@linux.ie" <airlied@linux.ie>, "hch@lst.de" <hch@lst.de>,
        "Ursulin, Tvrtko" <tvrtko.ursulin@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
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        "Auld, Matthew" <matthew.auld@intel.com>,
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Subject: Re: [PATCH v5] drm/i915: stop using swiotlb
Message-ID: <20220809185403.GB15307@lst.de>
References: <20220726153935.2272777-1-bob.beckett@collabora.com> <1160a7c31084ab2259088e4bfe88b41ad61c2bcc.camel@intel.com> <db9f787e-c3e4-d353-da57-80cb7a135d86@linux.intel.com>
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On Tue, Aug 09, 2022 at 12:36:50PM +0100, Tvrtko Ursulin wrote:
>
> Digging through git history at least running as Xen dom0 looks to have been 
> impacted, but commits such as abb0deacb5a6 ("drm/i915: Fallback to single 
> PAGE_SIZE segments for DMA remapping") are older and suggest problem was 
> generic. 1625e7e549c5 ("drm/i915: make compact dma scatter lists creation 
> work with SWIOTLB backend.") as well. So it looks it did work behind 
> swiotlb despite those missing calls you highlighted.

Hmm.  xen-swiotlb bounce buffers as soon as any single mapping that
straddles a Xen page size boundary.  Could the magic value there
somehow made all mappings small enough to just avoid bounce buffering
for Xen by more or less accident?