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Date:   Tue, 9 Aug 2022 20:51:57 +0200
From:   "hch@lst.de" <hch@lst.de>
To:     "Hellstrom, Thomas" <thomas.hellstrom@intel.com>
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        "joonas.lahtinen@linux.intel.com" <joonas.lahtinen@linux.intel.com>,
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Subject: Re: [PATCH v5] drm/i915: stop using swiotlb
Message-ID: <20220809185157.GA15307@lst.de>
References: <20220726153935.2272777-1-bob.beckett@collabora.com> <1160a7c31084ab2259088e4bfe88b41ad61c2bcc.camel@intel.com>
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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.