Re: [PATCH] drm/vkms: prime import support

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Hi Oleg,

First of all, thank you for your patch and for working in this issue.

A few comments inline.

On Thu, Jul 4, 2019 at 5:54 AM Oleg Vasilev <oleg.vasilev@xxxxxxxxx> wrote:
>
> Bring dmabuf sharing through implementing prime_import_sg_table callback.
> This will help to validate userspace conformance in prime configurations
> without using any actual hardware (e.g. in the cloud).
>
> Cc: Rodrigo Siqueira <rodrigosiqueiramelo@xxxxxxxxx>
> Cc: Haneen Mohammed <hamohammed.sa@xxxxxxxxx>
> Cc: Daniel Vetter <daniel@xxxxxxxx>
> Signed-off-by: Oleg Vasilev <oleg.vasilev@xxxxxxxxx>
> ---
>  drivers/gpu/drm/vkms/vkms_drv.c |  6 +++++
>  drivers/gpu/drm/vkms/vkms_drv.h |  9 +++++++
>  drivers/gpu/drm/vkms/vkms_gem.c | 46 +++++++++++++++++++++++++++++++++
>  3 files changed, 61 insertions(+)
>
> diff --git a/drivers/gpu/drm/vkms/vkms_drv.c b/drivers/gpu/drm/vkms/vkms_drv.c
> index cc53ef88a331..b71c16d9ca09 100644
> --- a/drivers/gpu/drm/vkms/vkms_drv.c
> +++ b/drivers/gpu/drm/vkms/vkms_drv.c
> @@ -10,6 +10,7 @@
>   */
>
>  #include <linux/module.h>
> +#include <linux/dma-buf.h>

Maybe I missed something, but I think you don't need to add this
include here since you already included it on vkms_gem.

>  #include <drm/drm_gem.h>
>  #include <drm/drm_atomic.h>
>  #include <drm/drm_atomic_helper.h>
> @@ -96,6 +97,8 @@ static struct drm_driver vkms_driver = {
>         .gem_vm_ops             = &vkms_gem_vm_ops,
>         .gem_free_object_unlocked = vkms_gem_free_object,
>         .get_vblank_timestamp   = vkms_get_vblank_timestamp,
> +       .prime_fd_to_handle     = drm_gem_prime_fd_to_handle,
> +       .gem_prime_import_sg_table = vkms_prime_import_sg_table,
>
>         .name                   = DRIVER_NAME,
>         .desc                   = DRIVER_DESC,
> @@ -147,6 +150,9 @@ static int __init vkms_init(void)
>
>         ret = drm_dev_init(&vkms_device->drm, &vkms_driver,
>                            &vkms_device->platform->dev);
> +
> +       dma_coerce_mask_and_coherent(vkms_device->drm.dev,
> +                                    DMA_BIT_MASK(64));

How about capture the return value from dma_coerce_mask_and_coherent()
and warn the user if something wrong happened? Something like:
ret = dma_coerce_mask_and_coherent(..);

if (ret)
  DRM_WARN("Failed to set dma mask");

Additionally, I would like to suggest you move this code above
drm_dev_init() since there's a return validation of ret in the below
if.

>         if (ret)
>                 goto out_unregister;
>
> diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
> index 12b4db7ac641..fb15101c8f3e 100644
> --- a/drivers/gpu/drm/vkms/vkms_drv.h
> +++ b/drivers/gpu/drm/vkms/vkms_drv.h
> @@ -126,6 +126,9 @@ struct drm_gem_object *vkms_gem_create(struct drm_device *dev,
>                                        u32 *handle,
>                                        u64 size);
>
> +struct vkms_gem_object *vkms_gem_create_private(struct drm_device *dev,
> +                                               u64 size);
> +
>  vm_fault_t vkms_gem_fault(struct vm_fault *vmf);
>
>  int vkms_dumb_create(struct drm_file *file, struct drm_device *dev,
> @@ -137,6 +140,12 @@ int vkms_gem_vmap(struct drm_gem_object *obj);
>
>  void vkms_gem_vunmap(struct drm_gem_object *obj);
>
> +/* Prime */
> +struct drm_gem_object *
> +vkms_prime_import_sg_table(struct drm_device *dev,
> +                          struct dma_buf_attachment *attach,
> +                          struct sg_table *sg);
> +
>  /* CRC Support */
>  const char *const *vkms_get_crc_sources(struct drm_crtc *crtc,
>                                         size_t *count);
> diff --git a/drivers/gpu/drm/vkms/vkms_gem.c b/drivers/gpu/drm/vkms/vkms_gem.c
> index 69048e73377d..a1b837460f63 100644
> --- a/drivers/gpu/drm/vkms/vkms_gem.c
> +++ b/drivers/gpu/drm/vkms/vkms_gem.c
> @@ -1,5 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0+
>
> +#include <linux/dma-buf.h>
>  #include <linux/shmem_fs.h>
>
>  #include "vkms_drv.h"
> @@ -117,6 +118,25 @@ struct drm_gem_object *vkms_gem_create(struct drm_device *dev,
>         return &obj->gem;
>  }
>
> +struct vkms_gem_object *vkms_gem_create_private(struct drm_device *dev,
> +                                               u64 size)
> +{

I did not fully get the idea behind this function in this patch, and
it looks like this function is never invoked. Am I right or I missed
something?

> +       struct vkms_gem_object *obj;
> +
> +       obj = kzalloc(sizeof(*obj), GFP_KERNEL);
> +
> +       if (!obj)
> +               return ERR_PTR(-ENOMEM);
> +
> +       size = roundup(size, PAGE_SIZE);
> +
> +       drm_gem_private_object_init(dev, &obj->gem, size);

Please, correct if I'm wrong, I'm trying to understand where this
function should be placed and when it is invoked. If I correctly
understood, you want to use this function for creating a separate gem
object to be used as a private area to be shared with another device,
am I right? If I am right, I'm wondering if we could reuse or update
some of the functions available in the vkms_gem. If I am wrong, I'm
wondering if we may have synchronization problems for handling
multiples mutex for accessing pages.  Could you help me here?

> +
> +       mutex_init(&obj->pages_lock);
> +
> +       return obj;
> +}
> +
>  int vkms_dumb_create(struct drm_file *file, struct drm_device *dev,
>                      struct drm_mode_create_dumb *args)
>  {
> @@ -217,3 +237,29 @@ int vkms_gem_vmap(struct drm_gem_object *obj)
>         mutex_unlock(&vkms_obj->pages_lock);
>         return ret;
>  }
> +
> +struct drm_gem_object *
> +vkms_prime_import_sg_table(struct drm_device *dev,
> +                          struct dma_buf_attachment *attach,
> +                          struct sg_table *sg)
> +{
> +       struct vkms_gem_object *obj;
> +       int npages;
> +
> +       obj = __vkms_gem_create(dev, attach->dmabuf->size);
> +       if (IS_ERR(obj))
> +               return ERR_CAST(obj);
> +
> +       npages = PAGE_ALIGN(attach->dmabuf->size) / PAGE_SIZE;
> +       DRM_DEBUG_PRIME("Importing %d  pages\n", npages);
> +
> +       obj->pages = kvmalloc_array(npages, sizeof(struct page *), GFP_KERNEL);

One question, why not use _get_pages() instead of using
kvmalloc_array? Use _get_pages has the advantage of already handling
some errors which may reduce the amount of code in this function.

> +       if (!obj->pages) {
> +               vkms_gem_free_object(&obj->gem);
> +               return ERR_PTR(-ENOMEM);
> +       }
> +
> +       drm_prime_sg_to_page_addr_arrays(sg, obj->pages, NULL,
> +                                        npages);

Why not add npage in the same line?

Thanks you again.

> +       return &obj->gem;
> +}
> --
> 2.22.0
>

-- 

Rodrigo Siqueira
https://siqueira.tech
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