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Date:   Wed, 24 Feb 2021 08:27:22 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Thomas Zimmermann <tzimmermann@suse.de>,
        Takashi Iwai <tiwai@suse.de>,
        Mathias Nyman <mathias.nyman@linux.intel.com>,
        airlied@linux.ie, gregkh@linuxfoundation.org,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Oliver Neukum <oneukum@suse.com>,
        Johan Hovold <johan@kernel.org>, hdegoede@redhat.com,
        Christoph Hellwig <hch@lst.de>,
        dri-devel@lists.freedesktop.org, stable@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        sean@poorly.run, christian.koenig@amd.com
Subject: Re: [PATCH v3] drm: Use USB controller's DMA mask when importing
 dmabufs
Message-ID: <20210224072722.GA32481@lst.de>
References: <20210223105842.27011-1-tzimmermann@suse.de> <s5hh7m2vqyd.wl-tiwai@suse.de> <f4452070-bab1-35ad-69aa-d020a4a3a5b7@suse.de> <20210223160054.GC1261797@rowland.harvard.edu>
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On Tue, Feb 23, 2021 at 11:00:54AM -0500, Alan Stern wrote:
> The impression I get is that Greg would like the USB core to export a 
> function which takes struct usb_interface * as argument and returns the 
> appropriate DMA mask value.  Then instead of messing around with USB 
> internals, drm_gem_prime_import_usb could just call this new function.
> 
> Adding such a utility function would be a sufficiently small change that 
> it could go into the -stable kernels with no trouble.

The DMA mask value on its own is not useful.  It needs to be paired with
a device that the dma API functions can be called on.