linux-kernel - Re: [PATCH v5] drm/i915: stop using swiotlb

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Date:   Tue, 9 Aug 2022 20:54:03 +0200
From:   "hch@....de" <hch@....de>
To:     Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
Cc:     "Hellstrom, Thomas" <thomas.hellstrom@...el.com>,
        "daniel@...ll.ch" <daniel@...ll.ch>,
        "joonas.lahtinen@...ux.intel.com" <joonas.lahtinen@...ux.intel.com>,
        "Vivi, Rodrigo" <rodrigo.vivi@...el.com>,
        "bob.beckett@...labora.com" <bob.beckett@...labora.com>,
        "jani.nikula@...ux.intel.com" <jani.nikula@...ux.intel.com>,
        "airlied@...ux.ie" <airlied@...ux.ie>, "hch@....de" <hch@....de>,
        "Ursulin, Tvrtko" <tvrtko.ursulin@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "kernel@...labora.com" <kernel@...labora.com>,
        "Auld, Matthew" <matthew.auld@...el.com>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>
Subject: Re: [PATCH v5] drm/i915: stop using swiotlb

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?