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SubjectRe: [PATCH v2 4/8] drm/virtio: Improve DMA API usage for shmem BOs
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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@collabora.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.

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