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
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