linux-kernel - Re: [PATCH v2 4/8] drm/virtio: Improve DMA API usage for shmem BOs

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Date:   Thu, 17 Mar 2022 04:09:03 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     David Airlie <airlied@...ux.ie>, Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Daniel Almeida <daniel.almeida@...labora.com>,
        Gert Wollny <gert.wollny@...labora.com>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Rob Herring <robh@...nel.org>,
        Steven Price <steven.price@....com>,
        Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Gustavo Padovan <gustavo.padovan@...labora.com>,
        Daniel Stone <daniel@...ishbar.org>,
        virtualization@...ts.linux-foundation.org,
        Dmitry Osipenko <digetx@...il.com>
Subject: Re: [PATCH v2 4/8] drm/virtio: Improve DMA API usage for shmem BOs

On 3/15/22 01:42, Dmitry Osipenko wrote:
> DRM API requires the DRM's driver to be backed with the device that can
> be used for generic DMA operations. The VirtIO-GPU device can't perform
> DMA operations if it uses PCI transport because PCI device driver creates
> a virtual VirtIO-GPU device that isn't associated with the PCI. Use PCI's
> GPU device for the DRM's device instead of the VirtIO-GPU device and drop
> DMA-related hacks from the VirtIO-GPU driver.
> 
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
> ---
>  drivers/gpu/drm/virtio/virtgpu_drv.c    | 22 +++++++---
>  drivers/gpu/drm/virtio/virtgpu_drv.h    |  5 +--
>  drivers/gpu/drm/virtio/virtgpu_kms.c    |  7 ++--
>  drivers/gpu/drm/virtio/virtgpu_object.c | 56 +++++--------------------
>  drivers/gpu/drm/virtio/virtgpu_vq.c     | 13 +++---
>  5 files changed, 37 insertions(+), 66 deletions(-)
> 
> diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.c b/drivers/gpu/drm/virtio/virtgpu_drv.c
> index 5f25a8d15464..8449dad3e65c 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_drv.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_drv.c
> @@ -46,9 +46,9 @@ static int virtio_gpu_modeset = -1;
>  MODULE_PARM_DESC(modeset, "Disable/Enable modesetting");
>  module_param_named(modeset, virtio_gpu_modeset, int, 0400);
>  
> -static int virtio_gpu_pci_quirk(struct drm_device *dev, struct virtio_device *vdev)
> +static int virtio_gpu_pci_quirk(struct drm_device *dev)

Somehow I missed that virtio_gpu_pci_quirk() contains comment telling
about why dev.parent isn't used for the DRM's device.

	/*
	 * Normally the drm_dev_set_unique() call is done by core DRM.
	 * The following comment covers, why virtio cannot rely on it.
	 *
	 * Unlike the other virtual GPU drivers, virtio abstracts the
	 * underlying bus type by using struct virtio_device.
	 *
	 * Hence the dev_is_pci() check, used in core DRM, will fail
	 * and the unique returned will be the virtio_device "virtio0",
	 * while a "pci:..." one is required.
	 *
	 * A few other ideas were considered:
	 * - Extend the dev_is_pci() check [in drm_set_busid] to
	 *   consider virtio.
	 *   Seems like a bigger hack than what we have already.
	 *
	 * - Point drm_device::dev to the parent of the virtio_device
	 *   Semantic changes:
	 *   * Using the wrong device for i2c, framebuffer_alloc and
	 *     prime import.
	 *   Visual changes:
	 *   * Helpers such as DRM_DEV_ERROR, dev_info, drm_printer,
	 *     will print the wrong information.
	 *
	 * We could address the latter issues, by introducing
	 * drm_device::bus_dev, ... which would be used solely for this.
	 *
	 * So for the moment keep things as-is, with a bulky comment
	 * for the next person who feels like removing this
	 * drm_dev_set_unique() quirk.
	 */

There is no I2C, nor prime import support and framebuffer_alloc hasn't
been used for years now. I guess prime import actually will want the
real device and not the virtio_device if GEM importing will be ever
supported by VirtIO-GPU driver. Apparently this comment was outdated a
long time ago and the "visual changes" aren't too important, the
messages now says "virtio-pci 0000:00:02.0" instead of "virtio_gpu virtio0".

I'll remove the comment and drm_dev_set_unique() quirk in v3.