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

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Message-ID: <20220809185157.GA15307@lst.de>
Date:   Tue, 9 Aug 2022 20:51:57 +0200
From:   "hch@....de" <hch@....de>
To:     "Hellstrom, Thomas" <thomas.hellstrom@...el.com>
Cc:     "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>,
        "tvrtko.ursulin@...ux.intel.com" <tvrtko.ursulin@...ux.intel.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 Mon, Aug 08, 2022 at 03:48:02PM +0000, Hellstrom, Thomas wrote:
> This whole thing looks a bit strange to me since with SWIOTLB actually
> used for i915, the driver should malfunction anyway as it doesn't do
> any dma_sync_sg_for_cpu() or dma_sync_sg_for_device(),

Yeah, I can't actually see any syncs in i915.

> and the driver
> assumes all coherent dma. Is that SWIOTLB=force kernel option still
> available?

Yes, although it is (and always has been) lower case swiotlb for the
option.

> Also, correct me if I'm wrong, but the original driver segment size
> appears to mean "the largest contiguous area that can be handled either
> by the device or the dma mapping layer" rather than the total space
> available for dma mappings? Not completely sure what
> dma_max_mapping_size() is returning, though?

dma_max_mapping_size is sort of both.  It is is the largest contigous
size, but we really should not go above that to avoid starvation.