Date: Thu, 27 Jun 2024 08:57:40 +0200
From: Christian König <christian.koenig@....com>
To: Jason-JH Lin (林睿祥) <Jason-JH.Lin@...iatek.com>,
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Subject: Re: [PATCH v5 2/9] scatterlist: Add a flag for the restricted memory

Am 27.06.24 um 05:21 schrieb Jason-JH Lin (林睿祥):
>
> On Wed, 2024-06-26 at 19:56 +0200, Daniel Vetter wrote:
> >   
> > External email : Please do not click links or open attachments until
> > you have verified the sender or the content.
> >  On Wed, Jun 26, 2024 at 12:49:02PM +0200, Christian König wrote:
> > > Am 26.06.24 um 10:05 schrieb Jason-JH Lin (林睿祥):
> > > > > > 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.
> > > 
> > > Why can't you get this information from userspace?
> > 
> > Same reason amd and i915/xe also pass this around internally in the
> > kernel, it's just that for those gpus the render and kms node are the
> > same
> > driver so this is easy.
> >

The reason I ask is that encryption here looks just like another 
parameter for the buffer, e.g. like format, stride, tilling etc..

So instead of this during buffer import:

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;

You can trivially say during use hey this buffer is encrypted.

At least that's my 10 mile high view, maybe I'm missing some extensive 
key exchange or something like that.


>   
> > But on arm you have split designs everywhere and dma-buf
> > import/export, so
> > something else is needed. And neither current kms uapi nor
> > protocols/extensions have provisions for this (afaik) because it
> > works on
> > the big gpus, and on android it's just hacked up with backchannels.
> > 
> > So yeah essentially I think we probably need something like this, as
> > much
> > as it sucks. I see it somewhat similar to handling pcip2pdma
> > limitations
> > in the kernel too.
> > 
> > Not sure where/how it should be handled though, and maybe I've missed
> > something around protocols, in which case I guess we should add some
> > secure buffer flags to the ADDFB2 ioctl.
>
> Thanks for your hint, I'll try to add the secure flag to the ADDFB2
> ioctl. If it works, I'll send the patch.

Yeah, exactly what I would suggest as well.

I'm not an expert for that part, but as far as I know we already have 
bunch of device specific tilling flags in there.

Adding an MTK_ENCRYPTED flag should be trivial.

Regards,
Christian.

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