linux-kernel - Re: [PATCH v2 1/3] usb: core: add dma-noncoherent buffer alloc and free API

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Message-ID: <aF5_EwSHHMOOMRv9@smile.fi.intel.com>
Date: Fri, 27 Jun 2025 14:22:59 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Xu Yang <xu.yang_2@....com>
Cc: ezequiel@...guardiasur.com.ar, mchehab@...nel.org,
	laurent.pinchart@...asonboard.com, hdegoede@...hat.com,
	gregkh@...uxfoundation.org, mingo@...nel.org, tglx@...utronix.de,
	viro@...iv.linux.org.uk, thomas.weissschuh@...utronix.de,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org, imx@...ts.linux.dev, jun.li@....com
Subject: Re: [PATCH v2 1/3] usb: core: add dma-noncoherent buffer alloc and
 free API

On Fri, Jun 27, 2025 at 06:19:37PM +0800, Xu Yang wrote:
> This will add usb_alloc_noncoherent() and usb_free_noncoherent()
> functions to support alloc and free buffer in a dma-noncoherent way.
> 
> To explicit manage the memory ownership for the kernel and device,
> this will also add usb_dma_noncoherent_sync_for_cpu/device() functions
> and call it at proper time.  The management requires the user save
> sg_table returned by usb_alloc_noncoherent() to urb->sgt.

...

> +/**
> + * usb_alloc_noncoherent - allocate dma-noncoherent buffer for URB_NO_xxx_DMA_MAP
> + * @dev: device the buffer will be used with
> + * @size: requested buffer size
> + * @mem_flags: affect whether allocation may block
> + * @dma: used to return DMA address of buffer
> + * @dir: dma transfer direction
> + * @table: used to return sg_table of allocated memory
> + *
> + * Return: Either null (indicating no buffer could be allocated), or the
> + * cpu-space pointer to a buffer that may be used to perform DMA to the
> + * specified device.  Such cpu-space buffers are returned along with the DMA
> + * address (through the pointer provided).

Return section should be last in the kernel-doc (this requirement is
documented).

> + * To explicit manage the memory ownership for the kernel vs the device by
> + * usb core, the user needs save sg_table to urb->sgt. Then usb core will
> + * do dma sync for cpu and device properly.
> + *
> + * When the buffer is no longer used, free it with usb_free_noncoherent().

Here and everywhere else in your series, pay respect to acronyms by using them
as acronyms:

dma --> DMA
cpu --> CPU
usb --> USB

and so on...


> + */
> +void *usb_alloc_noncoherent(struct usb_device *dev, size_t size,
> +			    gfp_t mem_flags, dma_addr_t *dma,
> +			    enum dma_data_direction dir,
> +			    struct sg_table **table)
> +{
> +	struct device *dmadev;
> +	struct sg_table *sgt;
> +	void *buffer;
> +
> +	if (!dev || !dev->bus)

When !dev case is possible?

> +		return NULL;
> +
> +	dmadev = bus_to_hcd(dev->bus)->self.sysdev;
> +
> +	sgt = dma_alloc_noncontiguous(dmadev, size, dir, mem_flags, 0);
> +	if (!sgt)
> +		return NULL;
> +
> +	buffer = dma_vmap_noncontiguous(dmadev, size, sgt);
> +	if (!buffer) {
> +		dma_free_noncontiguous(dmadev, size, sgt, dir);
> +		return NULL;
> +	}
> +
> +	*table = sgt;
> +	*dma = sg_dma_address(sgt->sgl);
> +
> +	return buffer;
> +}

...

> +void usb_free_noncoherent(struct usb_device *dev, size_t size,
> +			  void *addr, enum dma_data_direction dir,
> +			  struct sg_table *table)
> +{
> +	struct device *dmadev;
> +
> +	if (!dev || !dev->bus)

Ditto.

> +		return;
> +	if (!addr)
> +		return;
> +
> +	dmadev = bus_to_hcd(dev->bus)->self.sysdev;
> +	dma_vunmap_noncontiguous(dmadev, addr);
> +	dma_free_noncontiguous(dmadev, size, table, dir);
> +}

-- 
With Best Regards,
Andy Shevchenko