Re: [PATCH v7 11/12] firmware: qcom: scm: clarify the comment in qcom_scm_pas_init_image()

[Prasad Sodagudi]

On 2/17/2024 7:50 PM, Bjorn Andersson wrote:
> On Mon, Feb 05, 2024 at 07:28:09PM +0100, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
> >
> > The "memory protection" mechanism mentioned in the comment is the SHM
> > Bridge. This is also the reason why we do not convert this call to using
> > the TZ memory allocator.
> No, this mechanism predates shmbridge.
Yes. PIL calls are there for very long time and shm bridge concept is 
introduced later.
> > Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
> > Tested-by: Andrew Halaney <ahalaney@xxxxxxxxxx> # sc8280xp-lenovo-thinkpad-x13s
> > Tested-by: Deepti Jaggi <quic_djaggi@xxxxxxxxxxx> #sa8775p-ride
> > Reviewed-by: Elliot Berman <quic_eberman@xxxxxxxxxxx>
> > ---
> >   drivers/firmware/qcom/qcom_scm.c | 10 +++++++---
> >   1 file changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/firmware/qcom/qcom_scm.c b/drivers/firmware/qcom/qcom_scm.c
> > index 839773270a21..7ba5cff6e4e7 100644
> > --- a/drivers/firmware/qcom/qcom_scm.c
> > +++ b/drivers/firmware/qcom/qcom_scm.c
> > @@ -563,9 +563,13 @@ int qcom_scm_pas_init_image(u32 peripheral, const void *metadata, size_t size,
> >   	struct qcom_scm_res res;
> >   
> >   	/*
> > -	 * During the scm call memory protection will be enabled for the meta
> > -	 * data blob, so make sure it's physically contiguous, 4K aligned and
> > -	 * non-cachable to avoid XPU violations.
> What this is saying is that the memory will be made unaccessible to
> Linux, so it needs to be contiguous and aligned.

Based on my understanding,  this buffer has to be  physically 
contiguous, 4K aligned and non-cachable to avoid XPU violations.

We should keep this comment

> > +	 * During the SCM call the hypervisor will make the buffer containing
> > +	 * the program data into an SHM Bridge.
> Are you saying that the hypervisor will convert this memory to a
> shmbridge, and then pass it to TrustZone?
Yes.  Specifically for PIL calls hyp is creating shm bridge for 
buffers/regions on behalf of Linux/NS-EL1.
> > This is why we exceptionally
> > +	 * must not use the TrustZone memory allocator here as - depending on
> > +	 * Kconfig - it may already use the SHM Bridge mechanism internally.
> > +	 *
> "it may already"? You describe above that we shouldn't pass shmbridge
> memory, and the other case never deals with shmbridges. So, I think you
> can omit this part.
>
> > +	 * If we pass a buffer that is already part of an SHM Bridge to this
> > +	 * call, it will fail.
> Could this be because the consumer of this buffer operates in EL2, and
> not TZ?
These buffers are consumed by TZ only and not by EL2.  hyp creating the 
required bridges for pil buffers.
> Regards,
> Bjorn
>
> >   	 */
> >   	mdata_buf = dma_alloc_coherent(__scm->dev, size, &mdata_phys,
> >   				       GFP_KERNEL);
> > --
> > 2.40.1