Re: [PATCH 2/6] dmaengine: xilinx_dma: fix completion callback is not invoked for each DMA operation

[Andrea Merello]

On Wed, Jun 20, 2018 at 2:36 PM, Radhey Shyam Pandey <radheys@xxxxxxxxxx> wrote:
>> -----Original Message-----
>> From: dmaengine-owner@xxxxxxxxxxxxxxx [mailto:dmaengine-
>> owner@xxxxxxxxxxxxxxx] On Behalf Of Andrea Merello
>> Sent: Wednesday, June 20, 2018 2:07 PM
>> To: vkoul@xxxxxxxxxx; dan.j.williams@xxxxxxxxx; Michal Simek
>> <michals@xxxxxxxxxx>; Appana Durga Kedareswara Rao
>> <appanad@xxxxxxxxxx>; dmaengine@xxxxxxxxxxxxxxx
>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
>> Andrea Merello <andrea.merello@xxxxxxxxx>
>> Subject: [PATCH 2/6] dmaengine: xilinx_dma: fix completion callback is not
>> invoked for each DMA operation
>>
>> API specification says: "On completion of each DMA operation, the next in
>> queue is started and a tasklet triggered. The tasklet will then call the
>> client driver completion callback routine for notification, if set."
>>
>> Currently the driver keeps a "desc_pendingcount" counter of the total
>> descriptor pending, and it uses as IRQ coalesce threshold, as result it
>> only calls the CBs after ALL pending operations are completed, which is
>> wrong.
> I think IRQ coalescing enable/disable should be configurable.
> Performance related usecases will need this support.

I didn't intend this (only) wrt performances; my concern was mostly
wrt correctness. If my point of view is wrong then I'll drop this
patch from the series.

(.. I might respin it again in future: I had a patch wrt an old driver
version that allowed submitting new descriptors to the HW while the
DMA is running, and in this case disabling coalesce is needed i.e. in
order to submit a new empty buffer whenever the DMA finishes a
transfer without waiting the DMA to stop).

BTW, is there any dmaengine API suitable for setting interrupt coalesce?

>>
>> This patch uses disable IRQ coalesce and checks for the completion flag
>> for the descriptors (which is further divided in segments).
>>
>> Possibly a better optimization could be using proper IRQ coalesce
>> threshold to get an IRQ after all segments of the descriptors are done.
>> But we don't do that yet..
>>
>> NOTE: for now we do this only for AXI DMA, other DMA flavors are
>> untested/untouched.
>> This is loosely based on
>> commit 65df81a6dc74 ("xilinx_dma: IrqThreshold set incorrectly, unreliable.")
>> in my linux-4.6-zynq tree
> NOTE description doesn't help much.
>
>>
>> From: Jeremy Trimble [original patch]
>> Signed-off-by: Andrea Merello <andrea.merello@xxxxxxxxx>
>> ---
>>  drivers/dma/xilinx/xilinx_dma.c | 39 +++++++++++++++++++++------------
>>  1 file changed, 25 insertions(+), 14 deletions(-)
>>
>> diff --git a/drivers/dma/xilinx/xilinx_dma.c b/drivers/dma/xilinx/xilinx_dma.c
>> index a516e7ffef21..cf12f7147f07 100644
>> --- a/drivers/dma/xilinx/xilinx_dma.c
>> +++ b/drivers/dma/xilinx/xilinx_dma.c
>> @@ -164,6 +164,7 @@
>>  #define XILINX_DMA_CR_COALESCE_SHIFT 16
>>  #define XILINX_DMA_BD_SOP            BIT(27)
>>  #define XILINX_DMA_BD_EOP            BIT(26)
>> +#define XILINX_DMA_BD_CMPLT          BIT(31)
>>  #define XILINX_DMA_COALESCE_MAX              255
>>  #define XILINX_DMA_NUM_DESCS         255
>>  #define XILINX_DMA_NUM_APP_WORDS     5
>> @@ -1274,12 +1275,9 @@ static void xilinx_dma_start_transfer(struct
>> xilinx_dma_chan *chan)
>>
>>       reg = dma_ctrl_read(chan, XILINX_DMA_REG_DMACR);
>>
>> -     if (chan->desc_pendingcount <= XILINX_DMA_COALESCE_MAX) {
>> -             reg &= ~XILINX_DMA_CR_COALESCE_MAX;
>> -             reg |= chan->desc_pendingcount <<
>> -                               XILINX_DMA_CR_COALESCE_SHIFT;
>> -             dma_ctrl_write(chan, XILINX_DMA_REG_DMACR, reg);
>> -     }
>> +     reg &= ~XILINX_DMA_CR_COALESCE_MAX;
>> +     reg |= 1 << XILINX_DMA_CR_COALESCE_SHIFT;
>> +     dma_ctrl_write(chan, XILINX_DMA_REG_DMACR, reg);
>>
>>       if (chan->has_sg && !chan->xdev->mcdma)
>>               xilinx_write(chan, XILINX_DMA_REG_CURDESC,
>> @@ -1378,6 +1376,20 @@ static void xilinx_dma_complete_descriptor(struct
>> xilinx_dma_chan *chan)
>>               return;
>>
>>       list_for_each_entry_safe(desc, next, &chan->active_list, node) {
>> +             if (chan->xdev->dma_config->dmatype ==
>> XDMA_TYPE_AXIDMA) {
>> +                     /*
>> +                      * Check whether the last segment in this descriptor
>> +                      * has been completed.
>> +                      */
>> +                     const struct xilinx_axidma_tx_segment *const tail_seg
>> =
>> +                             list_last_entry(&desc->segments,
>> +                                             struct
>> xilinx_axidma_tx_segment,
>> +                                             node);
>> +
>> +                     /* we've processed all the completed descriptors */
>> +                     if (!(tail_seg->hw.status & XILINX_DMA_BD_CMPLT))
>> +                             break;
>> +             }
>>               list_del(&desc->node);
>>               if (!desc->cyclic)
>>                       dma_cookie_complete(&desc->async_tx);
>> @@ -1826,14 +1838,13 @@ static struct dma_async_tx_descriptor
>> *xilinx_dma_prep_slave_sg(
>>                                  struct xilinx_axidma_tx_segment, node);
>>       desc->async_tx.phys = segment->phys;
>>
>> -     /* For the last DMA_MEM_TO_DEV transfer, set EOP */
>> -     if (chan->direction == DMA_MEM_TO_DEV) {
>> -             segment->hw.control |= XILINX_DMA_BD_SOP;
>> -             segment = list_last_entry(&desc->segments,
>> -                                       struct xilinx_axidma_tx_segment,
>> -                                       node);
>> -             segment->hw.control |= XILINX_DMA_BD_EOP;
>> -     }
>> +     /* For the first transfer, set SOP */
>> +     segment->hw.control |= XILINX_DMA_BD_SOP;
>> +     /* For the last transfer, set EOP */
>> +     segment = list_last_entry(&desc->segments,
>> +                               struct xilinx_axidma_tx_segment,
>> +                               node);
>> +     segment->hw.control |= XILINX_DMA_BD_EOP;
>>
>>       return &desc->async_tx;
>>
>> --
>> 2.17.1
>>
>> --
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