From cf9610f9116d55c5a66ef9f1faecacabd93a9322 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Mon, 24 Aug 2026 08:46:03 -0700 Subject: Merge tag 'pinctrl-v7.3-1' of git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-pinctrl Pull pin control updates from Linus Walleij: "Core changes: - Use the non-blocking mux_state_try_select() in the generic MUX pin control back-end - Free pin maps on pinctrl_generic_to_map() failure in the core helpers New hardware pin control: - Qualcomm Maili TLMM SoC, PMG1110 PMIC, and Eliza LPASS LPI TLMM SoC (this is for the low power audio, LPASS = Low Power Audio SubSystem portions) - Upboard support on top of the Intel pin controller - Apple T6030 and T6031 support (just compatible strings) - Samsung Exynos 8855 SoC - Mediatek MT6858 SoC - Rockchip RK3308B and RV1106 SoCs - Airoha EN7523 and AN7563 SoCs Improvements: - Qualcomm TLMM GPIOs are unconditionally marked as wakeup capable - Qualcomm improvements on top of the Qualcomm PDC (power domain controller) irqchip improvements from tglx:s tree - Qualcomm IPQ806x and IPQ9650 non-urgent pin grop fixups - The NPCM8xx driver has been put into shape fixing many pin and group definitions - Some non-urgent Tegra 264 pin assignments are fixed up - Some non-urgent fixups to Airoha AN7581 and AN7583 registers and pin assignments, missing features etc" * tag 'pinctrl-v7.3-1' of git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-pinctrl: (130 commits) dt-bindings: pinctrl: Convert TI DA850 pupd to DT schema pinctrl: meson: sync some modify from A4 pinctrl: meson: a4: Add input enable pin configuration pinctrl: realtek: rtd1625: remove unused group name spdif_sel pinctrl: realtek: rtd1625: fix base_bit for VE4 GPIO 13 dt-bindings: pinctrl: microchip,pic32mzda-pinctrl: Convert to DT schema Revert "Merge branch 'ib-rsk7204' into devel" pinctrl: rockchip: Add RV1106 pinctrl support dt-bindings: pinctrl: rockchip: Add RV1106 compatible pinctrl: rockchip: Decode drive strength in the get function pinctrl: fix PINCTRL_GENERIC_MUX not always being selectable pinctrl: airoha: add support of an7563 SoC dt-bindings: pinctrl: airoha: add support of an7563 pin controller pinctrl: airoha: try to find chip scu node by phandle first pinctrl: airoha: add support of en7523 SoC dt-bindings: pinctrl: airoha: add support of en7523 pin controller pinctrl: airoha: an7583: add support for olt pinmux pinctrl: airoha: an7583: add support for pon_alt pinmux pinctrl: airoha: an7583: add support for npu_uart pinmux dt-bindings: pinctrl: airoha: an7583: add missed features ... --- Documentation/driver-api/usb/dma.rst | 110 +++++++++++++++++++++++++++++++++++ 1 file changed, 110 insertions(+) create mode 100644 Documentation/driver-api/usb/dma.rst (limited to 'Documentation/driver-api/usb/dma.rst') diff --git a/Documentation/driver-api/usb/dma.rst b/Documentation/driver-api/usb/dma.rst new file mode 100644 index 000000000..02f6825ff --- /dev/null +++ b/Documentation/driver-api/usb/dma.rst @@ -0,0 +1,110 @@ +USB DMA +~~~~~~~ + +In Linux 2.5 kernels (and later), USB device drivers have additional control +over how DMA may be used to perform I/O operations. The APIs are detailed +in the kernel usb programming guide (kerneldoc, from the source code). + +API overview +============ + +The big picture is that USB drivers can continue to ignore most DMA issues, +though they still must provide DMA-ready buffers (see +Documentation/core-api/dma-api-howto.rst). That's how they've worked through +the 2.4 (and earlier) kernels, or they can now be DMA-aware. + +DMA-aware usb drivers: + +- New calls enable DMA-aware drivers, letting them allocate dma buffers and + manage dma mappings for existing dma-ready buffers (see below). + +- URBs have an additional "transfer_dma" field, as well as a transfer_flags + bit saying if it's valid. (Control requests also have "setup_dma", but + drivers must not use it.) + +- "usbcore" will map this DMA address, if a DMA-aware driver didn't do + it first and set ``URB_NO_TRANSFER_DMA_MAP``. HCDs + don't manage dma mappings for URBs. + +- There's a new "generic DMA API", parts of which are usable by USB device + drivers. Never use dma_set_mask() on any USB interface or device; that + would potentially break all devices sharing that bus. + +Eliminating copies +================== + +It's good to avoid making CPUs copy data needlessly. The costs can add up, +and effects like cache-trashing can impose subtle penalties. + +- If you're doing lots of small data transfers from the same buffer all + the time, that can really burn up resources on systems which use an + IOMMU to manage the DMA mappings. It can cost MUCH more to set up and + tear down the IOMMU mappings with each request than perform the I/O! + + For those specific cases, USB has primitives to allocate less expensive + memory. They work like kmalloc and kfree versions that give you the right + kind of addresses to store in urb->transfer_buffer and urb->transfer_dma. + You'd also set ``URB_NO_TRANSFER_DMA_MAP`` in urb->transfer_flags:: + + void *usb_alloc_coherent (struct usb_device *dev, size_t size, + int mem_flags, dma_addr_t *dma); + + void usb_free_coherent (struct usb_device *dev, size_t size, + void *addr, dma_addr_t dma); + + Most drivers should **NOT** be using these primitives; they don't need + to use this type of memory ("dma-coherent"), and memory returned from + :c:func:`kmalloc` will work just fine. + + The memory buffer returned is "dma-coherent"; sometimes you might need to + force a consistent memory access ordering by using memory barriers. It's + not using a streaming DMA mapping, so it's good for small transfers on + systems where the I/O would otherwise thrash an IOMMU mapping. (See + Documentation/core-api/dma-api-howto.rst for definitions of "coherent" and + "streaming" DMA mappings.) + + Asking for 1/Nth of a page (as well as asking for N pages) is reasonably + space-efficient. + + On most systems the memory returned will be uncached, because the + semantics of dma-coherent memory require either bypassing CPU caches + or using cache hardware with bus-snooping support. While x86 hardware + has such bus-snooping, many other systems use software to flush cache + lines to prevent DMA conflicts. + +- Devices on some EHCI controllers could handle DMA to/from high memory. + + Unfortunately, the current Linux DMA infrastructure doesn't have a sane + way to expose these capabilities ... and in any case, HIGHMEM is mostly a + design wart specific to x86_32. So your best bet is to ensure you never + pass a highmem buffer into a USB driver. That's easy; it's the default + behavior. Just don't override it; e.g. with ``NETIF_F_HIGHDMA``. + + This may force your callers to do some bounce buffering, copying from + high memory to "normal" DMA memory. If you can come up with a good way + to fix this issue (for x86_32 machines with over 1 GByte of memory), + feel free to submit patches. + +Working with existing buffers +============================= + +Existing buffers aren't usable for DMA without first being mapped into the +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 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_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_sg_wait(struct usb_sg_request *io); + + void usb_sg_cancel(struct usb_sg_request *io); + + 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. -- cgit v1.3.1