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

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Date:   Tue, 18 Jun 2019 20:26:21 +0530
From:   Sricharan R <sricharan@...eaurora.org>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.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 Srini,

On 6/18/2019 8:20 PM, Srinivas Kandagatla wrote:
> 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?
> 
  So MAX_DESCRIPTORS is used in driver for masking head/tail pointers.
  That's why we have to pass MAX_DESCRIPTORS + 1 so that it works
  when the Macros does a size - 1

Regards,
 Sricharan

> 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;
>>>
>>

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