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

["Christian König" <christian.koenig@amd.com>]

Am 19.02.21 um 16:53 schrieb 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.

Yeah, I was assuming something like that would work.

But in this case the USB device should give the host controllers device 
structure to the dma_buf_attach function so that the sg_table can be 
filled in with DMA addresses properly.

Regards,
Christian.

>
> Alan Stern