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

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Message-ID: <1964d238-d56a-b9ea-cda8-0b9ff32481cb@quicinc.com>
Date: Fri, 10 Jan 2025 17:53:21 +0530
From: Md Sadre Alam <quic_mdalam@...cinc.com>
To: Stephan Gerhold <stephan.gerhold@...aro.org>
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 1/10/2025 5:37 PM, Stephan Gerhold wrote:
> 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.
Ok Thanks. will check once again this and post in next revision.
> 
> Thanks,
> Stephan