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-----Original Message-----
From: Gupta, Suraj <redacted>
Sent: Wednesday, July 23, 2025 5:19 PM
To: Vinod Koul <vkoul@kernel.org>
Cc: andrew+netdev@lunn.ch; davem@davemloft.net; kuba@kernel.org;
pabeni@redhat.com; Simek, Michal [off-list ref]; Pandey,
Radhey Shyam [off-list ref]; netdev@vger.kernel.org;
linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org;
dmaengine@vger.kernel.org; Katakam, Harini [off-list ref]
Subject: RE: [PATCH V2 1/4] dmaengine: Add support to configure and read
IRQ coalescing parameters

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Hi Vinod,

quoted

-----Original Message-----
From: Vinod Koul <vkoul@kernel.org>
Sent: Wednesday, July 23, 2025 1:00 PM
To: Gupta, Suraj <redacted>
Cc: andrew+netdev@lunn.ch; davem@davemloft.net; kuba@kernel.org;
pabeni@redhat.com; Simek, Michal [off-list ref]; Pandey,
Radhey Shyam [off-list ref];

netdev@vger.kernel.org;

quoted

linux-arm- kernel@lists.infradead.org; linux-kernel@vger.kernel.org;
dmaengine@vger.kernel.org; Katakam, Harini [off-list ref]
Subject: Re: [PATCH V2 1/4] dmaengine: Add support to configure and
read IRQ coalescing parameters

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On 10-07-25, 15:42, Suraj Gupta wrote:

quoted

Interrupt coalescing is a mechanism to reduce the number of hardware
interrupts triggered ether until a certain amount of work is
pending, or a timeout timer triggers. Tuning the interrupt coalesce
settings involves adjusting the amount of work and timeout delay.
Many DMA controllers support to configure coalesce count and delay.
Add support to configure them via dma_slave_config and read using
dma_slave_caps.

Signed-off-by: Suraj Gupta <redacted>
---
 include/linux/dmaengine.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
index bb146c5ac3e4..c7c1adb8e571 100644
--- a/include/linux/dmaengine.h
+++ b/include/linux/dmaengine.h

@@ -431,6 +431,9 @@ enum dma_slave_buswidth {
  * @peripheral_config: peripheral configuration for programming

peripheral

quoted

quoted

  * for dmaengine transfer
  * @peripheral_size: peripheral configuration buffer size
+ * @coalesce_cnt: Maximum number of transfers before receiving an

interrupt.

quoted

quoted

+ * @coalesce_usecs: How many usecs to delay an interrupt after a
+ transfer
+ * is completed.
  *
  * This struct is passed in as configuration data to a DMA engine
  * in order to set up a certain channel for DMA transport at runtime.

@@ -457,6 +460,8 @@ struct dma_slave_config {
      bool device_fc;
      void *peripheral_config;
      size_t peripheral_size;
+     u32 coalesce_cnt;
+     u32 coalesce_usecs;
 };

 /**

@@ -507,6 +512,9 @@ enum dma_residue_granularity {
  * @residue_granularity: granularity of the reported transfer residue
  * @descriptor_reuse: if a descriptor can be reused by client and
  * resubmitted multiple times
+ * @coalesce_cnt: Maximum number of transfers before receiving an

interrupt.

quoted

quoted

+ * @coalesce_usecs: How many usecs to delay an interrupt after a
+ transfer
+ * is completed.
  */
 struct dma_slave_caps {
      u32 src_addr_widths;

@@ -520,6 +528,8 @@ struct dma_slave_caps {
      bool cmd_terminate;
      enum dma_residue_granularity residue_granularity;
      bool descriptor_reuse;
+     u32 coalesce_cnt;
+     u32 coalesce_usecs;

Why not selectively set interrupts for the descriptor. The dma
descriptors are in order, so one a descriptor is notified and
complete, you can also complete the descriptors before that. I would
suggest to use that rather than define a new interface for this

--
~Vinod

The reason I used struct dma_slave_config to pass coalesce and delay
information to DMA driver is that the coalesce count is configured per
channel in AXI DMA channel control register[1].
AXI DMA IP doesn't have provision to set interrupt per descriptor[2].
I can explore other ways to pass this information via struct
dma_async_tx_descriptor or metadata, or any other way.
Please let me know your thoughts.

References:
[1]: https://docs.amd.com/r/en-US/pg021_axi_dma/MM2S_DMACR-MM2S-
DMA-Control-Register-Offset-00h ("IRQ Threshold" and "IRQ Delay" fields)
[2]: https://docs.amd.com/r/en-US/pg021_axi_dma/Scatter-Gather-
Descriptor

Thanks,
Suraj

Hi Vinod,

I've added a rationale above explaining why I opted to implement a new interface instead of using the interrupt flag in struct dma_async_tx_descriptor.
Please let me know your thoughts when you get a chance.

Regards,
Suraj