Re: [PATCH v3] dmaengine: qcom: bam_dma: Fix BAM_RIVISON register handling

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

On 1/21/2025 10:39 PM, Stephan Gerhold wrote:
> On Tue, Jan 21, 2025 at 02:42:41PM +0530, Md Sadre Alam wrote:
>> This patch resolves a bug from the previous commit where the
>> BAM_DESC_CNT_TRSHLD register was conditionally written based on BAM-NDP
>> mode. The issue was reading the BAM_REVISION register hanging if num-ees
>> was not zero, which occurs when the SoCs power on BAM remotely. So the
>> BAM_REVISION register read has been moved to inside if condition.
>>
>> Fixes: 57a7138d0627 ("dmaengine: qcom: bam_dma: Avoid writing unavailable register")
>> Reported-by: Georgi Djakov <djakov@kernel.org>
>> Link: https://lore.kernel.org/lkml/9ef3daa8-cdb1-49f2-8d19-a72d6210ff3a@kernel.org/
>> Signed-off-by: Md Sadre Alam <quic_mdalam@quicinc.com>
> 
> I'm afraid there are still two open problems here:
> 
>   1. In your original commit, you added the if (in_range(...)) checks to
>      make the BAM_DESC_CNT_TRSHLD register write conditional. With this
>      patch we only read the bam_revision for the !bdev->num_ees case.
>      This means that even if we have e.g. a remotely powered BAM-NDP,
>      we don't initialize BAM_DESC_CNT_TRSHLD anymore.
> 
>   2. Aside from BAM-NDP and BAM-Lite there is also plain "BAM". You
>      mentioned we should only skip the register write for BAM-Lite, but
>      the plain "BAM" isn't handled anywhere yet.
> 
> I would recommend inverting the in_range(...) checks to check for if
> (!in_range(BAM-LITE) rather than if (in_range(BAM-NDP)). This should
> also work for the plain "BAM" type. It will also avoid regressions if we
> don't read the bam_revision in the !bdev->num_ees case. (Although
> ideally you would lazily initialize the bam_revision to cover all the
> configurations.)
Thanks for explanation and suggestion. Will address all the above points
in next revision.
> 
> Thanks,
> Stephan
> 
>> ---
>>
>> Change in [v3]
>>
>> * Revised commit details
>>
>> Change in [v2]
>>
>> * Removed unnecessary if checks.
>> * Relocated the BAM_REVISION register read within the if condition.
>>
>> Change in [v1]
>>
>> * https://lore.kernel.org/lkml/1a5fc7e9-39fe-e527-efc3-1ea990bbb53b@quicinc.com/
>> * Posted initial fixup for BAM revision register read handling
>>   drivers/dma/qcom/bam_dma.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/dma/qcom/bam_dma.c b/drivers/dma/qcom/bam_dma.c
>> index c14557efd577..d227b4f5b6b9 100644
>> --- a/drivers/dma/qcom/bam_dma.c
>> +++ b/drivers/dma/qcom/bam_dma.c
>> @@ -1199,11 +1199,11 @@ 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)
>> -- 
>> 2.34.1
>>