linux-kernel - Re: [PATCH v3 1/3] usb: core: add dma-noncoherent buffer alloc and free API

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Message-ID: <198ddef7-8623-400f-8e2e-ffd6c4bfd2cf@rowland.harvard.edu>
Date: Wed, 2 Jul 2025 10:32:01 -0400
From: Alan Stern <stern@...land.harvard.edu>
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,
	andriy.shevchenko@...ux.intel.com, viro@...iv.linux.org.uk,
	thomas.weissschuh@...utronix.de, dafna.hirschfeld@...labora.com,
	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 v3 1/3] usb: core: add dma-noncoherent buffer alloc and
 free API

On Wed, Jul 02, 2025 at 07:02:20PM +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.
> 
> Signed-off-by: Xu Yang <xu.yang_2@....com>
> 
> ---
> Changes in v3:
>  - put Return section at the end of description
>  - correct some abbreviations
>  - remove usb_dma_noncoherent_sync_for_cpu() and
>    usb_dma_noncoherent_sync_for_device()
>  - do DMA sync in usb_hcd_map_urb_for_dma() and
>    usb_hcd_unmap_urb_for_dma()
>  - call flush_kernel_vmap_range() for OUT transfers
>    and invalidate_kernel_vmap_range() for IN transfers
> ---
>  drivers/usb/core/hcd.c | 19 ++++++++++
>  drivers/usb/core/usb.c | 80 ++++++++++++++++++++++++++++++++++++++++++
>  include/linux/usb.h    | 11 ++++++
>  3 files changed, 110 insertions(+)
> 
> diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c
> index c22de97432a0..e0fa6d6d273b 100644
> --- a/drivers/usb/core/hcd.c
> +++ b/drivers/usb/core/hcd.c
> @@ -1366,6 +1366,14 @@ void usb_hcd_unmap_urb_for_dma(struct usb_hcd *hcd, struct urb *urb)
>  				urb->transfer_buffer_length,
>  				dir);
>  
> +	if ((urb->transfer_flags & URB_NO_TRANSFER_DMA_MAP) &&
> +	    urb->sgt) {

Shouldn't this be "else if"?  There aren't any circumstances where a 
driver might want to do two DMA mappings for the same buffer, are there?

> +		dma_sync_sgtable_for_cpu(hcd->self.sysdev, urb->sgt, dir);
> +		if (dir == DMA_FROM_DEVICE)
> +			invalidate_kernel_vmap_range(urb->transfer_buffer,
> +						     urb->transfer_buffer_length);
> +	}
> +

Alan Stern