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Date:   Thu, 18 Feb 2021 16:24:25 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Thomas Zimmermann <tzimmermann@suse.de>
Cc:     Christoph Hellwig <hch@lst.de>,
        Greg KH <gregkh@linuxfoundation.org>,
        Daniel Vetter <daniel@ffwll.ch>,
        David Airlie <airlied@redhat.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        USB list <linux-usb@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Regression: 6eb0233ec2d0 ("usb: don't inherity DMA properties
 for USB devices")
Message-ID: <20210218152425.GA19470@lst.de>
References: <ef15869a-fe87-aef2-591d-2ca909a66217@suse.de> <20210218150746.GA18220@lst.de> <ef51698a-cd93-47b3-b79f-8c86a4c215fc@suse.de>
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On Thu, Feb 18, 2021 at 04:21:26PM +0100, Thomas Zimmermann wrote:
> Sure, it's at [1]. For udl, the dmabuf would need to be in system memory. 
> The driver creates urbs from the framebuffer content and sends them to the 
> device for displaying.
>
> My question is more: what's the best interface to do this? Is there example 
> code somewhere?

URBs do point to system memory most of the time.  I'm not a real USB
expert, but usb-storage or uas would be classic examples for drivers
that take arbitrary memory controller by another layer and do DMA to/from
that.