Re: [PATCH] firmware: qcom_scm: Give page_aligned size for dma api's

[Elliot Berman]

On Fri, May 17, 2024 at 01:02:56AM +0530, Mukesh Ojha wrote:
> If we disable CONFIG_ZONE_DMA32 to make the selection of DMA
> memory from higher 4GB range. dma_alloc_coherant() api usage
                                dma_alloc_coherent()
> inside qcom_scm_pas_init_image() which usage scm 32bit device
> will fail for size of data passed less than PAGE_SIZE and
> it will fallback to buddy pool to allocate memory from which
> will fail.

I interpreted this as:

When CONFIG_ZONE_DMA32 is disabled, dma_alloc_coherent() fails when size
is < PAGE_SIZE. qcom_scm_pas_init_image() will fail to allocate using
dma_alloc_coherent() and incorrectly fall back to buddy pool.

This justification seems incorrect to me. None of the other
dma_alloc_coherent() users are page-aligning their requests in scm
driver. Is something else going on?

> 
> Convert the size to aligned to PAGE_SIZE before it gets pass
> to dma_alloc_coherant(), so that it gets coherant memory in
     dma_alloc_coherent                    coherent
> lower 4GB from linux cma region.
> 
> Signed-off-by: Mukesh Ojha <quic_mojha@xxxxxxxxxxx>
> ---
>  drivers/firmware/qcom/qcom_scm.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/firmware/qcom/qcom_scm.c b/drivers/firmware/qcom/qcom_scm.c
> index 029ee5edbea6..6616048f1c33 100644
> --- a/drivers/firmware/qcom/qcom_scm.c
> +++ b/drivers/firmware/qcom/qcom_scm.c
> @@ -562,6 +562,7 @@ static void qcom_scm_set_download_mode(u32 dload_mode)
>  int qcom_scm_pas_init_image(u32 peripheral, const void *metadata, size_t size,
>  			    struct qcom_scm_pas_metadata *ctx)
>  {
> +	size_t page_aligned_size;
>  	dma_addr_t mdata_phys;
>  	void *mdata_buf;
>  	int ret;
> @@ -579,7 +580,8 @@ int qcom_scm_pas_init_image(u32 peripheral, const void *metadata, size_t size,
>  	 * data blob, so make sure it's physically contiguous, 4K aligned and
>  	 * non-cachable to avoid XPU violations.
>  	 */
> -	mdata_buf = dma_alloc_coherent(__scm->dev, size, &mdata_phys,
> +	page_aligned_size = PAGE_ALIGN(size + PAGE_SIZE);

Isn't PAGE_ALIGN(size) good enough? Why do you need to round up to the
2nd page? Maybe you thought PAGE_ALIGN was PAGE_ALIGN_DOWN ?

> +	mdata_buf = dma_alloc_coherent(__scm->dev, page_aligned_size, &mdata_phys,
>  				       GFP_KERNEL);
>  	if (!mdata_buf)
>  		return -ENOMEM;
> @@ -604,11 +606,11 @@ int qcom_scm_pas_init_image(u32 peripheral, const void *metadata, size_t size,
>  
>  out:
>  	if (ret < 0 || !ctx) {
> -		dma_free_coherent(__scm->dev, size, mdata_buf, mdata_phys);
> +		dma_free_coherent(__scm->dev, page_aligned_size, mdata_buf, mdata_phys);
>  	} else if (ctx) {
>  		ctx->ptr = mdata_buf;
>  		ctx->phys = mdata_phys;
> -		ctx->size = size;
> +		ctx->size = page_aligned_size;
>  	}
>  
>  	return ret ? : res.result[0];
> -- 
> 2.7.4
> 
>