Re: [PATCH] dmaengine: qcom: bam_dma: Fix BAM_RIVISON register handling

Md Sadre Alam <quic_mdalam@xxxxxxxxxxx> · Fri, 10 Jan 2025 16:38:18 +0530

On 1/10/2025 3:45 PM, Stephan Gerhold wrote:
On Fri, Jan 10, 2025 at 10:44:09AM +0530, Md Sadre Alam wrote:
This patch fixes a bug introduced in the previous commit where the
BAM_DESC_CNT_TRSHLD register was conditionally written based on BAM-NDP
mode. Additionally, it addresses an issue where reading the BAM_REVISION
register hangs if num-ees is not zero. A check has been added to prevent
this.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 57a7138d0627 ("dmaengine: qcom: bam_dma: Avoid writing unavailable register")
Reported-by: Georgi Djakov <djakov@xxxxxxxxxx>
Link: https://lore.kernel.org/lkml/9ef3daa8-cdb1-49f2-8d19-a72d6210ff3a@xxxxxxxxxx/
Signed-off-by: Md Sadre Alam <quic_mdalam@xxxxxxxxxxx>
---
  drivers/dma/qcom/bam_dma.c | 23 ++++++++++++++++-------
  1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/drivers/dma/qcom/bam_dma.c b/drivers/dma/qcom/bam_dma.c
index c14557efd577..2b88b27f2f91 100644
--- a/drivers/dma/qcom/bam_dma.c
+++ b/drivers/dma/qcom/bam_dma.c
@@ -445,11 +445,15 @@ 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->num_ees && 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));
  
+	if (bdev->num_ees && !bdev->bam_revision)
+		writel_relaxed(DEFAULT_CNT_THRSHLD, bam_addr(bdev, 0,
+							     BAM_DESC_CNT_TRSHLD));
+
  	/* Enable default set of h/w workarounds, ie all except BAM_FULL_PIPE */
  	writel_relaxed(BAM_CNFG_BITS_DEFAULT, bam_addr(bdev, 0, BAM_CNFG_BITS));
  
@@ -1006,10 +1010,14 @@ 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->num_ees && in_range(bdev->bam_revision, BAM_NDP_REVISION_START,
+					       BAM_NDP_REVISION_END))
  			writel_relaxed(maxburst,
  				       bam_addr(bdev, 0, BAM_DESC_CNT_TRSHLD));
+
+		if (bdev->num_ees && !bdev->bam_revision)
+			writel_relaxed(DEFAULT_CNT_THRSHLD, bam_addr(bdev, 0,
+								     BAM_DESC_CNT_TRSHLD));

I guess you meant writel_relaxed(maxburst, ...) here?

This patch is quite confusing. We shouldn't duplicate the register
writes here just to have different handling for if (bdev->num_ees) and
if (!bdev->num_ees).

Also, num-ees is unrelated to the question if the BAM is BAM-NDP or
BAM-Lite. Typically we specify qcom,num-ees in the device tree for a BAM
if the BAM is either:

  - Controlled remotely (= powered on and initialized outside of Linux)
    This is the case for the SLIMbus BAM Georgi mentioned.

  - Powered remotely (= powered on outside of Linux, but must be
    initialized inside Linux)

Reading BAM_REVISION in these cases will hang in bam_init(), because we
cannot guarantee the BAM is already powered on when the bam_dma driver
is being loaded in Linux. We need to delay reading the register until
the BAM is up.

Given that these writes happen only for the !bdev->controlled_remotely
case, you could fix this more cleanly by reading the BAM revision inside
bam_reset().
Thank you for review and suggestion. Will clean up in next revision.

Thanks,
Alam.







