[PATCH] drm/prime: Only call dma_map_sgtable() for devices with DMA support

[Alan Stern]

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