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Date: Fri, 26 Feb 2021 11:19:13 +0100
From: Greg KH <gregkh@linuxfoundation.org>
To: Thomas Zimmermann <tzimmermann@suse.de>
Subject: Re: [PATCH v5] drm: Use USB controller's DMA mask when importing
 dmabufs
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References: <20210226092648.4584-1-tzimmermann@suse.de>
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Cc: Pavel Machek <pavel@ucw.cz>, airlied@linux.ie,
 Daniel Vetter <daniel.vetter@ffwll.ch>, Christoph Hellwig <hch@lst.de>,
 hdegoede@redhat.com, stern@rowland.harvard.edu,
 dri-devel@lists.freedesktop.org, stable@vger.kernel.org, sean@poorly.run,
 christian.koenig@amd.com
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On Fri, Feb 26, 2021 at 10:26:47AM +0100, Thomas Zimmermann wrote:
> USB devices cannot perform DMA and hence have no dma_mask set in their
> device structure. Therefore importing dmabuf into a USB-based driver
> fails, which breaks joining and mirroring of display in X11.
> =

> For USB devices, pick the associated USB controller as attachment device.
> This allows the DRM import helpers to perform the DMA setup. If the DMA
> controller does not support DMA transfers, we're out of luck and cannot
> import. Our current USB-based DRM drivers don't use DMA, so the actual
> DMA device is not important.
> =

> Drivers should use DRM_GEM_SHMEM_DROVER_OPS_USB to initialize their
> instance of struct drm_driver.
> =

> Tested by joining/mirroring displays of udl and radeon un der Gnome/X11.
> =

> v5:
> 	* provide a helper for USB interfaces (Alan)
> 	* add FIXME item to documentation and TODO list (Daniel)
> v4:
> 	* implement workaround with USB helper functions (Greg)
> 	* use struct usb_device->bus->sysdev as DMA device (Takashi)
> v3:
> 	* drop gem_create_object
> 	* use DMA mask of USB controller, if any (Daniel, Christian, Noralf)
> v2:
> 	* move fix to importer side (Christian, Daniel)
> 	* update SHMEM and CMA helpers for new PRIME callbacks
> =

> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> Fixes: 6eb0233ec2d0 ("usb: don't inherity DMA properties for USB devices")
> Tested-by: Pavel Machek <pavel@ucw.cz>
> Acked-by: Christian K=F6nig <christian.koenig@amd.com>
> Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: <stable@vger.kernel.org> # v5.10+
> ---
>  Documentation/gpu/todo.rst         | 15 ++++++++++
>  drivers/gpu/drm/drm_prime.c        | 45 ++++++++++++++++++++++++++++++
>  drivers/gpu/drm/tiny/gm12u320.c    |  2 +-
>  drivers/gpu/drm/udl/udl_drv.c      |  2 +-
>  drivers/usb/core/usb.c             | 31 ++++++++++++++++++++
>  include/drm/drm_gem_shmem_helper.h | 16 +++++++++++
>  include/drm/drm_prime.h            |  5 ++++
>  include/linux/usb.h                | 24 ++++++++++++++++
>  8 files changed, 138 insertions(+), 2 deletions(-)
> =

> diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst
> index f872d3d33218..c185e0a2951e 100644
> --- a/Documentation/gpu/todo.rst
> +++ b/Documentation/gpu/todo.rst
> @@ -617,6 +617,21 @@ Contact: Daniel Vetter
>  =

>  Level: Intermediate
>  =

> +Remove automatic page mapping from dma-buf importing
> +----------------------------------------------------
> +
> +When importing dma-bufs, the dma-buf and PRIME frameworks automatically =
map
> +imported pages into the importer's DMA area. This is a problem for USB d=
evices,
> +which do not support DMA operations. By default, importing fails for USB
> +devices. USB-based drivers work around this problem by employing
> +drm_gem_prime_import_usb(). To fix the issue, automatic page mappings sh=
ould
> +be removed from the buffer-sharing code.
> +
> +Contact: Thomas Zimmermann <tzimmermann@suse.de>, Daniel Vetter
> +
> +Level: Advanced
> +
> +
>  Better Testing
>  =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>  =

> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index 2a54f86856af..59013bb1cd4b 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -29,6 +29,7 @@
>  #include <linux/export.h>
>  #include <linux/dma-buf.h>
>  #include <linux/rbtree.h>
> +#include <linux/usb.h>
>  =

>  #include <drm/drm.h>
>  #include <drm/drm_drv.h>
> @@ -1055,3 +1056,47 @@ void drm_prime_gem_destroy(struct drm_gem_object *=
obj, struct sg_table *sg)
>  	dma_buf_put(dma_buf);
>  }
>  EXPORT_SYMBOL(drm_prime_gem_destroy);
> +
> +/**
> + * drm_gem_prime_import_usb - helper library implementation of the impor=
t callback for USB devices
> + * @dev: drm_device to import into
> + * @dma_buf: dma-buf object to import
> + *
> + * This is an implementation of drm_gem_prime_import() for USB-based dev=
ices.
> + * USB devices cannot perform DMA directly. This function selects the US=
B host
> + * controller as DMA device instead. Drivers can use this as their
> + * &drm_driver.gem_prime_import implementation.
> + *
> + * See also drm_gem_prime_import().
> + *
> + * FIXME: The dma-buf framework expects to map the exported pages into
> + *        the importer's DMA area. USB devices don't support DMA, and
> + *        importing would fail. Foir the time being, this function provi=
des
> + *        a workaround by using the USB controller's DMA area. The real
> + *        solution is to remove page-mapping operations from the dma-buf
> + *        framework.
> + *
> + * Returns: A GEM object on success, or a pointer-encoder errno value ot=
herwise.
> + */
> +#ifdef CONFIG_USB
> +struct drm_gem_object *drm_gem_prime_import_usb(struct drm_device *dev,
> +						struct dma_buf *dma_buf)
> +{
> +	struct device *dmadev;
> +	struct drm_gem_object *obj;
> +
> +	if (!dev_is_usb(dev->dev))
> +		return ERR_PTR(-ENODEV);

I have resisted the "dev_is_*()" type of function for USB for a long
time now, and I really don't want to add it now.

The driver core explicitly was not created with RTI (run type
identification), but over time it has been slowly added on a per-bus
basis for various reasons, some good and others not good.

In this function, why would a drm device that was NOT a usb device ever
call it?  Because of that, I don't think dev_is_usb() is needed at all,
just don't call this function unless it really is a USB device.

If you need help enforcing it, add a 'struct usb_interface *' to the
function parameters but please don't create this function.

Hm, looks like we have 2 extern definitions of usb_bus_type in different
.h files, I should go fix that up now to try to prevent this type of
thing in the first place...

thanks,

greg k-h
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