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

[Md Sadre Alam <quic_mdalam@quicinc.com>]

On 2/10/2025 2:14 PM, Stephan Gerhold wrote:
> On Fri, Feb 07, 2025 at 12:17:33PM -0800, 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>
> 
> This patch fixes the most critical regression (the bus hang), but the
> in_range(..., BAM_NDP) checks are also wrong. They do not consider the
> plain "BAM" type where the register is apparently also available. The
> check should be !in_range(..., BAM_LITE) instead to fix this.
> 
> I mentioned this twice to Md Sadre Alam [1, 2], but a fix was
> unfortunately never sent for that part of the regression.

I apologize for the delay. I was attending to a family member's medical 
emergency and couldn't address this sooner. I will test and post a new 
revision as soon as possible.

Thanks
Alam

> 
> I think we should take Caleb's patch and revert the entire patch for the
> 6.14 cycle. There are several incorrect assumptions in the original
> patch, it will be easier to review a fixed version with the full diff,
> rather than looking at incremental fixups.
> 
> On a somewhat related note, I'm working on a patch series for bam_dma to
> clean up the handling of remotely controlled BAMs. It will make it more
> clear when it's safe to access BAM registers and when not, and should
> allow reading the revision also for remotely controlled BAMs. This would
> avoid the need for all these if (!bdev->bam_revision) checks.
> 
> Thanks,
> Stephan
> 
> [1]: https://lore.kernel.org/linux-arm-msm/Z4D2jQNNW94qGIlv@linaro.org/
> [2]: https://lore.kernel.org/linux-arm-msm/Z4_U19_QyH2RJvKW@linaro.org/
> 
>> ---
>>   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,
>> -- 
>> Bjorn Andersson <bjorn.andersson@oss.qualcomm.com>
>>