linux-kernel - Re: [PATCH v12 10/11] drm/virtio: Support memory shrinking

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Date:   Tue, 7 Mar 2023 21:25:13 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        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>,
        Gustavo Padovan <gustavo.padovan@...labora.com>,
        Daniel Stone <daniel@...ishbar.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Qiang Yu <yuq825@...il.com>,
        Steven Price <steven.price@....com>,
        Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>,
        Rob Herring <robh@...nel.org>
Cc:     intel-gfx@...ts.freedesktop.org, kernel@...labora.com,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v12 10/11] drm/virtio: Support memory shrinking

On 3/7/23 13:42, Thomas Zimmermann wrote:
> Hi
> 
> Am 05.03.23 um 23:10 schrieb Dmitry Osipenko:
> [...]
>>         *bo_ptr = bo;
>> diff --git a/drivers/gpu/drm/virtio/virtgpu_plane.c
>> b/drivers/gpu/drm/virtio/virtgpu_plane.c
>> index 4c09e313bebc..3f21512ff153 100644
>> --- a/drivers/gpu/drm/virtio/virtgpu_plane.c
>> +++ b/drivers/gpu/drm/virtio/virtgpu_plane.c
>> @@ -238,20 +238,32 @@ static int virtio_gpu_plane_prepare_fb(struct
>> drm_plane *plane,
>>       struct virtio_gpu_device *vgdev = dev->dev_private;
>>       struct virtio_gpu_framebuffer *vgfb;
>>       struct virtio_gpu_object *bo;
>> +    int err;
>>         if (!new_state->fb)
>>           return 0;
>>         vgfb = to_virtio_gpu_framebuffer(new_state->fb);
>>       bo = gem_to_virtio_gpu_obj(vgfb->base.obj[0]);
>> -    if (!bo || (plane->type == DRM_PLANE_TYPE_PRIMARY &&
>> !bo->guest_blob))
>> +
>> +    if (virtio_gpu_is_shmem(bo)) {
> 
> Not really a problem with this patchset, but having such branches looks
> like a bug in the driver's GEM design. Whatever your GEM object needs or
> does, it should be hidden in the implementation. Why is virtio doing this?

There is another "VRAM" VirtIO-GPU BO type that doesn't implement the
pin/unpin callbacks. Perhaps another option was to add the callbacks.

>> +        err = drm_gem_pin(&bo->base.base);
> 
> As the driver uses GEM SHMEM, please call drm_gem_shmem_object_pin()
> directly and remove patch [09/11]. Same goes for the _unpin call below.
> 
> The problem with generic pinning interfaces is that there's no way of
> specifying where to pin to BO.  The problem is most apparent with TTM,
> where hardware often has multiple locations were buffer can be placed
> (VRAM, GART, system memory). So it's really a detail between the driver
> and the memory manager.
> 
> These generic internal GEM interfaces should only be used by DRM core
> and helpers. Drivers should use their memory manager's interface.

I'll switch to use drm_gem_shmem_object_pin() directly in v13, maybe add
virtio_gpu_gem_pin() helper for that. Thanks!

-- 
Best regards,
Dmitry