Re: [PATCH v3 14/15] firmware: qcom: scm: clarify the comment in qcom_scm_pas_init_image()

[Andrew Halaney]

On Mon, Oct 09, 2023 at 05:34:26PM +0200, 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 TM mem allocator.

s/TM/TZ/ ?

> 
> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
> ---
>  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..8a2475ced10a 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.
> +	 * During the SCM call the TrustZone will make the buffer containing
> +	 * the program data into an SHM Bridge. 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.
> +	 *
> +	 * If we pass a buffer that is already part of an SHM Bridge to this
> +	 * call, it will fail.

I can at least confirm this matches my testing results in v2, fwiw.

I guess you could use the Kconfig and conditionally use the TZ mem
allocator if !SHMBridge, but I don't know if its worth the if statements
or not.

Bummer, I can't think of a beautiful way to unify this outside of a
dedicated non SHM bridge pool for this...

>  	 */
>  	mdata_buf = dma_alloc_coherent(__scm->dev, size, &mdata_phys,
>  				       GFP_KERNEL);
> -- 
> 2.39.2
>