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From:   Randy Li <ayaka@soulik.info>
To:     linux-usb@vger.kernel.org
Cc:     Randy Li <ayaka@soulik.info>, stern@rowland.harvard.edu,
        gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org,
        corbet@lwn.net, linux-doc@vger.kernel.org
Subject: [PATCH 2/2] docs: driver-api: usb: update dma info
Date:   Fri, 15 Sep 2023 01:23:24 +0800
Message-ID: <20230914172336.18761-3-ayaka@soulik.info>
In-Reply-To: <20230914172336.18761-1-ayaka@soulik.info>
References: <20230914172336.18761-1-ayaka@soulik.info>
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We should not hide the recommend APIs in a obscure place.

Signed-off-by: Randy Li <ayaka@soulik.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