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Date:   Fri, 19 Feb 2021 10:53:28 -0500
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Christian =?iso-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>
Cc:     Thomas Zimmermann <tzimmermann@suse.de>, daniel@ffwll.ch,
        airlied@linux.ie, maarten.lankhorst@linux.intel.com,
        mripard@kernel.org, sumit.semwal@linaro.org,
        gregkh@linuxfoundation.org, dri-devel@lists.freedesktop.org,
        noralf@tronnes.org, Christoph Hellwig <hch@lst.de>,
        Johan Hovold <johan@kernel.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        "Ahmed S. Darwish" <a.darwish@linutronix.de>,
        Mathias Nyman <mathias.nyman@linux.intel.com>,
        Oliver Neukum <oneukum@suse.com>,
        Felipe Balbi <balbi@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, stable@vger.kernel.org
Subject: Re: [PATCH] drm/prime: Only call dma_map_sgtable() for devices with
 DMA support
Message-ID: <20210219155328.GA1111829@rowland.harvard.edu>
References: <20210219134014.7775-1-tzimmermann@suse.de>
 <02a45c11-fc73-1e5a-3839-30b080950af8@amd.com>
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On Fri, Feb 19, 2021 at 02:45:54PM +0100, Christian König wrote:
> Well as far as I can see this is a relative clear NAK.
> 
> When a device can't do DMA and has no DMA mask then why it is requesting an
> sg-table in the first place?

This may not be important for your discussion, but I'd like to give an 
answer to the question -- at least, for the case of USB.

A USB device cannot do DMA and has no DMA mask.  Nevertheless, if you 
want to send large amounts of bulk data to/from a USB device then using 
an SG table is often a good way to do it.  The reason is simple: All 
communication with a USB device has to go through a USB host controller, 
and many (though not all) host controllers _can_ do DMA and _do_ have a 
DMA mask.

The USB mass-storage and uas drivers in particular make heavy use of 
this mechanism.

Alan Stern