Re: [PATCH 22/22] usb: document that URB transfer_buffer should be aligned

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Mauro,

Thank you for the patch.

On Wednesday 29 Mar 2017 15:54:21 Mauro Carvalho Chehab wrote:
> Several host controllers, commonly found on ARM, like dwc2,
> require buffers that are CPU-word aligned for they to work.
> 
> Failing to do that will cause random troubles at the caller
> drivers, causing them to fail.
> 
> Document it.
> 
> Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
> ---
>  Documentation/driver-api/usb/URB.rst | 18 ++++++++++++++++++
>  drivers/usb/core/message.c           | 15 +++++++++++++++
>  include/linux/usb.h                  | 18 ++++++++++++++++++
>  3 files changed, 51 insertions(+)
> 
> diff --git a/Documentation/driver-api/usb/URB.rst
> b/Documentation/driver-api/usb/URB.rst index d9ea6a3996e7..b83b557e9891
> 100644
> --- a/Documentation/driver-api/usb/URB.rst
> +++ b/Documentation/driver-api/usb/URB.rst
> @@ -274,6 +274,24 @@ If you specify your own start frame, make sure it's
> several frames in advance of the current frame.  You might want this model
> if you're synchronizing ISO data with some other event stream.
> 
> +.. note::
> +
> +   Several host drivers require that the ``transfer_buffer`` to be aligned
> +   with the CPU word size (e. g. DWORD for 32 bits, QDWORD for 64 bits).

Is it the CPU word size or the DMA transfer size ? I assume the latter, and I 
wouldn't be surprised if the alignment requirement was 32-bit on at least some 
of the 64-bit platforms.

> +   It is up to USB drivers should ensure that they'll only pass buffers
> +   with such alignments.
> +
> +   Please also notice that, due to such restriction, the host driver

s/notice/note/ (and below as well) ?

> +   may also override PAD bytes at the end of the ``transfer_buffer``, up to
> the
> +   size of the CPU word.

"May" is quite weak here. If some host controller drivers require buffers to 
be aligned, then it's an API requirement, and all buffers must be aligned. I'm 
not even sure I would mention that some host drivers require it, I think we 
should just state that the API requires buffers to be aligned.

> +   Please notice that ancillary routines that transfer URBs, like
> +   usb_control_msg() also have such restriction.
> +
> +   Such word alignment condition is normally ensured if the buffer is
> +   allocated with kmalloc(), but this may not be the case if the driver
> +   allocates a bigger buffer and point to a random place inside it.
> +
> 
>  How to start interrupt (INT) transfers?
>  =======================================
> diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
> index 4c38ea41ae96..1662a4446475 100644
> --- a/drivers/usb/core/message.c
> +++ b/drivers/usb/core/message.c
> @@ -128,6 +128,21 @@ static int usb_internal_control_msg(struct usb_device
> *usb_dev, * make sure your disconnect() method can wait for it to complete.
> Since you * don't have a handle on the URB used, you can't cancel the
> request. *
> + * .. note::
> + *
> + *   Several host drivers require that the @data buffer to be aligned
> + *   with the CPU word size (e. g. DWORD for 32 bits, QDWORD for 64 bits).
> + *   It is up to USB drivers should ensure that they'll only pass buffers
> + *   with such alignments.
> + *
> + *   Please also notice that, due to such restriction, the host driver
> + *   may also override PAD bytes at the end of the @data buffer, up to the
> + *   size of the CPU word.
> + *
> + *   Such word alignment condition is normally ensured if the buffer is
> + *   allocated with kmalloc(), but this may not be the case if the driver
> + *   allocates a bigger buffer and point to a random place inside it.
> + *
>   * Return: If successful, the number of bytes transferred. Otherwise, a
> negative * error number.
>   */
> diff --git a/include/linux/usb.h b/include/linux/usb.h
> index 7e68259360de..8b5ad6624708 100644
> --- a/include/linux/usb.h
> +++ b/include/linux/usb.h
> @@ -1373,6 +1373,24 @@ typedef void (*usb_complete_t)(struct urb *);
>   * capable, assign NULL to it, so that usbmon knows not to use the value.
>   * The setup_packet must always be set, so it cannot be located in highmem.
> *
> + * .. note::
> + *
> + *   Several host drivers require that the @transfer_buffer to be aligned
> + *   with the CPU word size (e. g. DWORD for 32 bits, QDWORD for 64 bits).
> + *   It is up to USB drivers should ensure that they'll only pass buffers
> + *   with such alignments.
> + *
> + *   Please also notice that, due to such restriction, the host driver
> + *   may also override PAD bytes at the end of the @transfer_buffer, up to
> the + *   size of the CPU word.
> + *
> + *   Please notice that ancillary routines that start URB transfers, like
> + *   usb_control_msg() also have such restriction.
> + *
> + *   Such word alignment condition is normally ensured if the buffer is
> + *   allocated with kmalloc(), but this may not be the case if the driver
> + *   allocates a bigger buffer and point to a random place inside it.
> + *

Couldn't we avoid three copies of the same text ? The chance they will get 
out-of-sync is quite high.

>   * Initialization:
>   *
>   * All URBs submitted must initialize the dev, pipe, transfer_flags (may be

-- 
Regards,

Laurent Pinchart