linux-kernel - Re: [PATCH] dmaengine: qcom-bam: fix circular buffer handling

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Date:   Tue, 18 Jun 2019 15:49:49 +0100
From:   Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
To:     Sricharan R <sricharan@...eaurora.org>, vkoul@...nel.org
Cc:     dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] dmaengine: qcom-bam: fix circular buffer handling

Hi Sricharan,

On 18/06/2019 08:13, Sricharan R wrote:
> Hi Srini,
> 
> On 6/14/2019 7:50 PM, Srinivas Kandagatla wrote:
>> For some reason arguments to most of the circular buffers
>> macros are used in reverse, tail is used for head and vice versa.
>>
>> This leads to bam thinking that there is an extra descriptor at the
>> end and leading to retransmitting descriptor which was not scheduled
>> by any driver. This happens after MAX_DESCRIPTORS (4096) are scheduled
>> and done, so most of the drivers would not notice this, unless they are
>> heavily using bam dma. Originally found this issue while testing
>> SoundWire over SlimBus on DB845c which uses DMA very heavily for
>> read/writes.
>>
>> Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
>> ---
>>   drivers/dma/qcom/bam_dma.c | 9 ++++-----
>>   1 file changed, 4 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/dma/qcom/bam_dma.c b/drivers/dma/qcom/bam_dma.c
>> index cb860cb53c27..43d7b0a9713a 100644
>> --- a/drivers/dma/qcom/bam_dma.c
>> +++ b/drivers/dma/qcom/bam_dma.c
>> @@ -350,8 +350,8 @@ static const struct reg_offset_data bam_v1_7_reg_info[] = {
>>   #define BAM_DESC_FIFO_SIZE	SZ_32K
>>   #define MAX_DESCRIPTORS (BAM_DESC_FIFO_SIZE / sizeof(struct bam_desc_hw) - 1)
>>   #define BAM_FIFO_SIZE	(SZ_32K - 8)
>> -#define IS_BUSY(chan)	(CIRC_SPACE(bchan->tail, bchan->head,\
>> -			 MAX_DESCRIPTORS + 1) == 0)
>> +#define IS_BUSY(chan)	(CIRC_SPACE(bchan->head, bchan->tail,\
>> +			 MAX_DESCRIPTORS) == 0)
>>   
>>   struct bam_chan {
>>   	struct virt_dma_chan vc;
>> @@ -806,7 +806,7 @@ static u32 process_channel_irqs(struct bam_device *bdev)
>>   		offset /= sizeof(struct bam_desc_hw);
>>   
>>   		/* Number of bytes available to read */
>> -		avail = CIRC_CNT(offset, bchan->head, MAX_DESCRIPTORS + 1);
>> +		avail = CIRC_CNT(bchan->head, offset, MAX_DESCRIPTORS);
>>
>   one question, so MAX_DESCRIPTORS is already a mask,
>      #define MAX_DESCRIPTORS (BAM_DESC_FIFO_SIZE / sizeof(struct bam_desc_hw) - 1)
> 
>   CIRC_CNT/SPACE macros also does a size - 1, so would it not be a problem if we
>   just pass MAX_DESCRIPTORS ?

Thanks for looking at this,
TBH, usage of CIRC_* macros is only valid for power-of-2 buffers,
In bam case MAX_DESCRIPTORS is 4095.
Am really not sure why 8 bytes have been removed from fifo data buffer size.
So basically usage of these macros is incorrect in bam case, this need 
to be fixed properly.

Do you agree?

Vinod, can you hold off with this patch, I will try to find some time 
this week to cook up a better patch removing the usage of these macros.



thanks,
srini

> 
> Regards,
>   Sricharan
>    
>>   		list_for_each_entry_safe(async_desc, tmp,
>>   					 &bchan->desc_list, desc_node) {
>> @@ -997,8 +997,7 @@ static void bam_start_dma(struct bam_chan *bchan)
>>   			bam_apply_new_config(bchan, async_desc->dir);
>>   
>>   		desc = async_desc->curr_desc;
>> -		avail = CIRC_SPACE(bchan->tail, bchan->head,
>> -				   MAX_DESCRIPTORS + 1);
>> +		avail = CIRC_SPACE(bchan->head, bchan->tail, MAX_DESCRIPTORS);
>>   
>>   		if (async_desc->num_desc > avail)
>>   			async_desc->xfer_len = avail;
>>
>