linux-kernel - Re: [PATCH v4] dmaengine: qcom: bam_dma: Avoid writing unavailable register

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Message-ID: <Z4ENabbDjT8kfpQF@linaro.org>
Date: Fri, 10 Jan 2025 13:07:05 +0100
From: Stephan Gerhold <stephan.gerhold@...aro.org>
To: Md Sadre Alam <quic_mdalam@...cinc.com>
Cc: vkoul@...nel.org, robin.murphy@....com, u.kleine-koenig@...libre.com,
	martin.petersen@...cle.com, fenghua.yu@...el.com,
	av2082000@...il.com, linux-arm-msm@...r.kernel.org,
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
	quic_mmanikan@...cinc.com, quic_srichara@...cinc.com,
	quic_varada@...cinc.com
Subject: Re: [PATCH v4] dmaengine: qcom: bam_dma: Avoid writing unavailable
 register

On Fri, Jan 10, 2025 at 05:29:29PM +0530, Md Sadre Alam wrote:
> On 1/10/2025 3:59 PM, Stephan Gerhold wrote:
> > On Fri, Dec 20, 2024 at 03:12:03PM +0530, Md Sadre Alam wrote:
> > > Avoid writing unavailable register in BAM-Lite mode.
> > > BAM_DESC_CNT_TRSHLD register is unavailable in BAM-Lite
> > > mode. Its only available in BAM-NDP mode. So only write
> > > this register for clients who is using BAM-NDP.
> > > 
> > > Signed-off-by: Md Sadre Alam <quic_mdalam@...cinc.com>
> > 
> > What are we actually fixing here? Which platform is affected? Is there a
> > crash, reset, or incorrect behavior?
> On SDX75, QPIC use BAM-Lite and as per HW description this
> BAM_DESC_CNT_TRSHLD register is not available, and writing to this
> SDX75 was hanging.
> > 
> > We have had this code for years without reported issues, with both
> > BAM-NDP and BAM-Lite instances. The register documentation on APQ8016E
> > documents the BAM_DESC_CNT_TRSHLD register even for the BAM-Lite
> > instance. There is a comment that it doesn't apply to BAM-Lite, but I
> > would expect the written value just ends up being ignored in that case.
> With older xPU it was being ignored but with new xPU its hanging. HW
> team suggested don't write this register for BAM-Lite mode since its not
> available.
> > 

OK, thanks for the explanation.

> > Also, there is not just BAM-NDP and BAM-Lite, but also plain "BAM". What
> > about that one? Should we write to BAM_DESC_CNT_TRSHLD?
> Apart from BAM-Lite this register available in all the BAM

Please check again if we need to check for additional revision numbers
for the non-NDP BAM types then. Or alternatively, change the check to
write the register on if (!BAM-Lite) instead of if (BAM-NDP). That might
be easier.

Thanks,
Stephan