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Date: Thu, 21 Jun 2018 13:46:12 +0000
From: Radhey Shyam Pandey <radheys@...inx.com>
To: "andrea.merello@...il.com" <andrea.merello@...il.com>
CC: "vkoul@...nel.org" <vkoul@...nel.org>,
"dan.j.williams@...el.com" <dan.j.williams@...el.com>,
Michal Simek <michals@...inx.com>,
Appana Durga Kedareswara Rao <appanad@...inx.com>,
"dmaengine@...r.kernel.org" <dmaengine@...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>
Subject: RE: [PATCH 2/6] dmaengine: xilinx_dma: fix completion callback is not
invoked for each DMA operation
> -----Original Message-----
> From: Andrea Merello [mailto:andrea.merello@...il.com]
> Sent: Wednesday, June 20, 2018 7:02 PM
> To: Radhey Shyam Pandey <radheys@...inx.com>
> Cc: vkoul@...nel.org; dan.j.williams@...el.com; Michal Simek
> <michals@...inx.com>; Appana Durga Kedareswara Rao
> <appanad@...inx.com>; dmaengine@...r.kernel.org; linux-arm-
> kernel@...ts.infradead.org; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH 2/6] dmaengine: xilinx_dma: fix completion callback is not
> invoked for each DMA operation
>
> On Wed, Jun 20, 2018 at 2:36 PM, Radhey Shyam Pandey
> <radheys@...inx.com> wrote:
> >> -----Original Message-----
> >> From: dmaengine-owner@...r.kernel.org [mailto:dmaengine-
> >> owner@...r.kernel.org] On Behalf Of Andrea Merello
> >> Sent: Wednesday, June 20, 2018 2:07 PM
> >> To: vkoul@...nel.org; dan.j.williams@...el.com; Michal Simek
> >> <michals@...inx.com>; Appana Durga Kedareswara Rao
> >> <appanad@...inx.com>; dmaengine@...r.kernel.org
> >> Cc: linux-arm-kernel@...ts.infradead.org; linux-kernel@...r.kernel.org;
> >> Andrea Merello <andrea.merello@...il.com>
> >> 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.
>
If coalescing is enabled driver will know DMA completion when
all packet counts are processed. So I think it's correct implementation.
Making interrupt coalescing configurable will address all usecases.
> (.. 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@...il.com>
> >> ---
> >> 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
> >>
> >> --
> >> To unsubscribe from this list: send the line "unsubscribe dmaengine" in
> >> the body of a message to majordomo@...r.kernel.org
> >> More majordomo info at http://vger.kernel.org/majordomo-info.html
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