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Date: Wed, 26 Jun 2024 08:05:21 +0000
From: Jason-JH Lin (林睿祥) <Jason-JH.Lin@...iatek.com>
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	Youlin Pei (裴友林) <youlin.pei@...iatek.com>
Subject: Re: [PATCH v5 2/9] scatterlist: Add a flag for the restricted memory

> > 
> > Here is the expected protected content buffer flow in DRM:
> > 1) userspace allocates a dma-buf FD from the "restricted_mtk_cma"
> >  by
> > DMA_HEAP_IOCTL_ALLOC.
> > 2) userspace imports that dma-buf into the device using prime for
> > the
> > drm_file.
> > 3) userspace uses the already implemented driver import code for
> > the
> > special cases of protected content buffer.
>  
> What is so special on that case?

The special case simply means the the protected content buffer.

> 
> > In the step 3), we need to verify the dma-buf is allocated from
> > "restricted_mtk_cma", but there is no way to pass the secure flag
> >  or
> > private data from userspace to the import interface in DRM driver.
>  
> Why do you need to verify that?

I need to know the imported buffer is allocated from restricted cma and
mark it as a secure buffer in mediatek-drm driver. Then, I will add
some configuration to the hardware if the buffer is secure buffer, so
that it can get the permission to access the secure buffer.

> 
> > So I can only verify it like this now:
> > struct drm_gem_object *mtk_gem_prime_import_sg_table(struct
> > drm_device
> > *dev, struct dma_buf_attachment *attach, struct sg_table *sg)
> > {
> >     struct mtk_gem_obj *mtk_gem;
> > 
> >     /* check if the entries in the sg_table are contiguous */
> >     if (drm_prime_get_contiguous_size(sg) < attach->dmabuf->size) {
> >         DRM_ERROR("sg_table is not contiguous");
> >         return ERR_PTR(-EINVAL);
> >     }
> >     mtk_gem = mtk_gem_init(dev, attach->dmabuf->size);
> >     if (IS_ERR(mtk_gem))
> >         return ERR_CAST(mtk_gem);
> > 
> > +   mtk_gem->secure = (!strncmp(attach->dmabuf->exp_name,
> >  "restricted",
> > 10));
> >     mtk_gem->dma_addr = sg_dma_address(sg->sgl);
> >     mtk_gem->size = attach->dmabuf->size;
> >     mtk_gem->sg = sg;
> > 
> >     return &mtk_gem->base;
> > }
>  
> Complete NAK from my side to that approach. Importing of a DMA-buf
> should be independent of the exporter.
> 
> What you could do is to provide the secure buffer from a device and
> not a device heap.
> 

You mean I should allocate buffer in mediate-drm driver not userspace?
I just have modified this to userspace by the comment here:

https://patchwork.kernel.org/project/linux-mediatek/patch/20240403102701.369-3-shawn.sung@mediatek.com/#25806766

> > I think I have the same problem as the ECC_FLAG mention in:
> > 
> > 
https://lore.kernel.org/linux-media/20240515-dma-buf-ecc-heap-v1-0-54cbbd049511@kernel.org/
> > 
> > I think it would be better to have the user configurable private
> > information in dma-buf, so all the drivers who have the same
> > requirement can get their private information from dma-buf directly
> > and
> > no need to change or add the interface.
> > 
> > What's your opinion in this point?
>  
> Well of hand I don't see the need for that.
> 
> What happens if you get a non-secure buffer imported in your secure
> device?

We use the same mediatek-drm driver for secure and non-secure buffer.
If non-secure buffer imported to mediatek-drm driver, it's go to the
normal flow with normal hardware settings.

We use different configurations to make hardware have different
permission to access the buffer it should access.

So if we can't get the information of "the buffer is allocated from
restricted_mtk_cma" when importing the buffer into the driver, we won't
be able to configure the hardware correctly.

Regards,
Jason-JH.Lin

> 
> Regards,
> Christian.
> 
> > Regards,
> > Jason-JH.Lin
> > 
> > > Regards,
> > > Christian.
> > 
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