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Message-ID: <20230914172336.18761-3-ayaka@soulik.info>
Date: Fri, 15 Sep 2023 01:23:24 +0800
From: Randy Li <ayaka@...lik.info>
To: linux-usb@...r.kernel.org
Cc: Randy Li <ayaka@...lik.info>, stern@...land.harvard.edu,
gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
corbet@....net, linux-doc@...r.kernel.org
Subject: [PATCH 2/2] docs: driver-api: usb: update dma info
We should not hide the recommend APIs in a obscure place.
Signed-off-by: Randy Li <ayaka@...lik.info>
---
Documentation/driver-api/usb/dma.rst | 48 +++++++---------------------
1 file 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.
--
2.41.0
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