linux-kernel - RE: [PATCH V2 1/4] dmaengine: Add support to configure and read IRQ coalescing parameters

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Message-ID:
 <BL3PR12MB657195840F55252FBD319C95C93EA@BL3PR12MB6571.namprd12.prod.outlook.com>
Date: Mon, 25 Aug 2025 06:17:38 +0000
From: "Gupta, Suraj" <Suraj.Gupta2@....com>
To: Vinod Koul <vkoul@...nel.org>
CC: "andrew+netdev@...n.ch" <andrew+netdev@...n.ch>, "davem@...emloft.net"
	<davem@...emloft.net>, "kuba@...nel.org" <kuba@...nel.org>,
	"pabeni@...hat.com" <pabeni@...hat.com>, "Simek, Michal"
	<michal.simek@....com>, "Pandey, Radhey Shyam" <radhey.shyam.pandey@....com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "dmaengine@...r.kernel.org"
	<dmaengine@...r.kernel.org>, "Katakam, Harini" <harini.katakam@....com>
Subject: RE: [PATCH V2 1/4] dmaengine: Add support to configure and read IRQ
 coalescing parameters

[Public]

> -----Original Message-----
> From: Gupta, Suraj <Suraj.Gupta2@....com>
> Sent: Wednesday, July 23, 2025 5:19 PM
> To: Vinod Koul <vkoul@...nel.org>
> Cc: andrew+netdev@...n.ch; davem@...emloft.net; kuba@...nel.org;
> pabeni@...hat.com; Simek, Michal <michal.simek@....com>; Pandey,
> Radhey Shyam <radhey.shyam.pandey@....com>; netdev@...r.kernel.org;
> linux-arm-kernel@...ts.infradead.org; linux-kernel@...r.kernel.org;
> dmaengine@...r.kernel.org; Katakam, Harini <harini.katakam@....com>
> Subject: RE: [PATCH V2 1/4] dmaengine: Add support to configure and read
> IRQ coalescing parameters
>
> Caution: This message originated from an External Source. Use proper
> caution when opening attachments, clicking links, or responding.
>
>
> [Public]
>
> Hi Vinod,
>
> > -----Original Message-----
> > From: Vinod Koul <vkoul@...nel.org>
> > Sent: Wednesday, July 23, 2025 1:00 PM
> > To: Gupta, Suraj <Suraj.Gupta2@....com>
> > Cc: andrew+netdev@...n.ch; davem@...emloft.net; kuba@...nel.org;
> > pabeni@...hat.com; Simek, Michal <michal.simek@....com>; Pandey,
> > Radhey Shyam <radhey.shyam.pandey@....com>;
> netdev@...r.kernel.org;
> > linux-arm- kernel@...ts.infradead.org; linux-kernel@...r.kernel.org;
> > dmaengine@...r.kernel.org; Katakam, Harini <harini.katakam@....com>
> > Subject: Re: [PATCH V2 1/4] dmaengine: Add support to configure and
> > read IRQ coalescing parameters
> >
> > Caution: This message originated from an External Source. Use proper
> > caution when opening attachments, clicking links, or responding.
> >
> >
> > On 10-07-25, 15:42, Suraj Gupta wrote:
> > > 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 <suraj.gupta2@....com>
> > > ---
> > >  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
> > >   * for dmaengine transfer
> > >   * @peripheral_size: peripheral configuration buffer size
> > > + * @coalesce_cnt: Maximum number of transfers before receiving an
> interrupt.
> > > + * @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.
> > > + * @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