Hi 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.
It's semi-related. One idea we had for speeding up transfers is to use the USB controller's DMA functionality and allocate framebuffers accordingly. It needs a bit of work in the DRM memory-management code IIRC. And we do some internal modifications if the frambuffer's data, so direct forwarding from renderer to USB controller is not always possible.
Best regards Thomas
The USB mass-storage and uas drivers in particular make heavy use of this mechanism. Alan Stern
-- Thomas Zimmermann Graphics Driver Developer SUSE Software Solutions Germany GmbH Maxfeldstr. 5, 90409 Nürnberg, Germany (HRB 36809, AG Nürnberg) Geschäftsführer: Felix Imendörffer
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