Re: [PATCH] dmaengine: qcom: bam_dma: Avoid accessing BAM_REVISION on remote BAM

[Sam Day <me@samcday.com>]

Hello Bjorn,

On Fri Feb 7, 2025 at 9:17 PM CET, Bjorn Andersson wrote:
> Commit '57a7138d0627 ("dmaengine: qcom: bam_dma: Avoid writing
> unavailable register")' made this read unconditional, in order to
> identify if the instance is BAM-NDP or BAM-Lite.
> But the BAM_REVISION register is not accessible on remotely managed BAM
> instances and attempts to access it causes the system to crash.
>
> Move the access back to be conditional and expand the checks that was
> introduced to restore the old behavior when no revision information is
> available.
>
> Fixes: 57a7138d0627 ("dmaengine: qcom: bam_dma: Avoid writing unavailable register")
> Reported-by: Georgi Djakov <djakov@kernel.org>
> Closes: https://lore.kernel.org/lkml/9ef3daa8-cdb1-49f2-8d19-a72d6210ff3a@kernel.org/
> Signed-off-by: Bjorn Andersson <bjorn.andersson@oss.qualcomm.com>
> ---
>  drivers/dma/qcom/bam_dma.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/dma/qcom/bam_dma.c b/drivers/dma/qcom/bam_dma.c
> index c14557efd577..d42d913492a8 100644
> --- a/drivers/dma/qcom/bam_dma.c
> +++ b/drivers/dma/qcom/bam_dma.c
> @@ -445,8 +445,8 @@ static void bam_reset(struct bam_device *bdev)
>  	writel_relaxed(val, bam_addr(bdev, 0, BAM_CTRL));
>  
>  	/* set descriptor threshold, start with 4 bytes */
> -	if (in_range(bdev->bam_revision, BAM_NDP_REVISION_START,
> -		     BAM_NDP_REVISION_END))
> +	if (!bdev->bam_revision ||
> +	    in_range(bdev->bam_revision, BAM_NDP_REVISION_START, BAM_NDP_REVISION_END))
>  		writel_relaxed(DEFAULT_CNT_THRSHLD,
>  			       bam_addr(bdev, 0, BAM_DESC_CNT_TRSHLD));
>  
> @@ -1006,8 +1006,8 @@ static void bam_apply_new_config(struct bam_chan *bchan,
>  			maxburst = bchan->slave.src_maxburst;
>  		else
>  			maxburst = bchan->slave.dst_maxburst;
> -		if (in_range(bdev->bam_revision, BAM_NDP_REVISION_START,
> -			     BAM_NDP_REVISION_END))
> +		if (!bdev->bam_revision ||
> +		    in_range(bdev->bam_revision, BAM_NDP_REVISION_START, BAM_NDP_REVISION_END))
>  			writel_relaxed(maxburst,
>  				       bam_addr(bdev, 0, BAM_DESC_CNT_TRSHLD));
>  	}
> @@ -1199,11 +1199,12 @@ static int bam_init(struct bam_device *bdev)
>  	u32 val;
>  
>  	/* read revision and configuration information */
> -	val = readl_relaxed(bam_addr(bdev, 0, BAM_REVISION));
> -	if (!bdev->num_ees)
> +	if (!bdev->num_ees) {
> +		val = readl_relaxed(bam_addr(bdev, 0, BAM_REVISION));
>  		bdev->num_ees = (val >> NUM_EES_SHIFT) & NUM_EES_MASK;
>  
> -	bdev->bam_revision = val & REVISION_MASK;
> +		bdev->bam_revision = val & REVISION_MASK;
> +	}
>  
>  	/* check that configured EE is within range */
>  	if (bdev->ee >= bdev->num_ees)
>
> ---
> base-commit: 2014c95afecee3e76ca4a56956a936e23283f05b
> change-id: 20250207-bam-read-fix-2b31297d3fa1
>
> Best regards,

Thank you, this fixes the early synchronous external abort in
bam_dma_probe on my msm8916 device.

Tested-by: Sam Day <me@samcday.com>

Regards,
-Sam