diff options
author | Randy Li <ayaka@soulik.info> | 2023-09-14 20:23:24 +0300 |
---|---|---|
committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2023-10-05 10:35:12 +0300 |
commit | 44ceac8c92daa2e08ba402c6609293cef2969093 (patch) | |
tree | 5b3af5dcf16925a2054e5996e74c4687f5cf5bde /Documentation/driver-api/usb | |
parent | 1cf56299f9bc7d4b8e1e39af08f01d6380e28173 (diff) | |
download | linux-44ceac8c92daa2e08ba402c6609293cef2969093.tar.xz |
docs: driver-api: usb: update dma info
We should not hide the recommend APIs in a obscure place.
Signed-off-by: Randy Li <ayaka@soulik.info>
Link: https://lore.kernel.org/r/20230914172336.18761-3-ayaka@soulik.info
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'Documentation/driver-api/usb')
-rw-r--r-- | Documentation/driver-api/usb/dma.rst | 48 |
1 files changed, 11 insertions, 37 deletions
diff --git a/Documentation/driver-api/usb/dma.rst b/Documentation/driver-api/usb/dma.rst index d32c27e11b90..02f6825ff830 100644 --- a/Documentation/driver-api/usb/dma.rst +++ b/Documentation/driver-api/usb/dma.rst @@ -93,44 +93,18 @@ DMA address space of the device. However, most buffers passed to your driver can safely be used with such DMA mapping. (See the first section of Documentation/core-api/dma-api-howto.rst, titled "What memory is DMA-able?") -- When you're using scatterlists, you can map everything at once. On some - systems, this kicks in an IOMMU and turns the scatterlists into single - DMA transactions:: +- When you have the scatterlists which have been mapped for the USB controller, + you could use the new ``usb_sg_*()`` calls, which would turn scatterlist + into URBs:: - int usb_buffer_map_sg (struct usb_device *dev, unsigned pipe, - struct scatterlist *sg, int nents); + int usb_sg_init(struct usb_sg_request *io, struct usb_device *dev, + unsigned pipe, unsigned period, struct scatterlist *sg, + int nents, size_t length, gfp_t mem_flags); - void usb_buffer_dmasync_sg (struct usb_device *dev, unsigned pipe, - struct scatterlist *sg, int n_hw_ents); + void usb_sg_wait(struct usb_sg_request *io); - void usb_buffer_unmap_sg (struct usb_device *dev, unsigned pipe, - struct scatterlist *sg, int n_hw_ents); + void usb_sg_cancel(struct usb_sg_request *io); - It's probably easier to use the new ``usb_sg_*()`` calls, which do the DMA - mapping and apply other tweaks to make scatterlist i/o be fast. - -- Some drivers may prefer to work with the model that they're mapping large - buffers, synchronizing their safe re-use. (If there's no re-use, then let - usbcore do the map/unmap.) Large periodic transfers make good examples - here, since it's cheaper to just synchronize the buffer than to unmap it - each time an urb completes and then re-map it on during resubmission. - - These calls all work with initialized urbs: ``urb->dev``, ``urb->pipe``, - ``urb->transfer_buffer``, and ``urb->transfer_buffer_length`` must all be - valid when these calls are used (``urb->setup_packet`` must be valid too - if urb is a control request):: - - struct urb *usb_buffer_map (struct urb *urb); - - void usb_buffer_dmasync (struct urb *urb); - - void usb_buffer_unmap (struct urb *urb); - - The calls manage ``urb->transfer_dma`` for you, and set - ``URB_NO_TRANSFER_DMA_MAP`` so that usbcore won't map or unmap the buffer. - They cannot be used for setup_packet buffers in control requests. - -Note that several of those interfaces are currently commented out, since -they don't have current users. See the source code. Other than the dmasync -calls (where the underlying DMA primitives have changed), most of them can -easily be commented back in if you want to use them. + When the USB controller doesn't support DMA, the ``usb_sg_init()`` would try + to submit URBs in PIO way as long as the page in scatterlists is not in the + Highmem, which could be very rare in modern architectures. |