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Message-ID: <aP/BCfg2NnPut5IG@lizhi-Precision-Tower-5810>
Date: Mon, 27 Oct 2025 14:59:21 -0400
From: Frank Li <Frank.li@....com>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Marco Felsch <m.felsch@...gutronix.de>, robh@...nel.org,
Vinod Koul <vkoul@...nel.org>, Shawn Guo <shawnguo@...nel.org>,
Sascha Hauer <s.hauer@...gutronix.de>,
Pengutronix Kernel Team <kernel@...gutronix.de>,
Fabio Estevam <festevam@...il.com>,
Jiada Wang <jiada_wang@...tor.com>, dmaengine@...r.kernel.org,
imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] dmaengine: add device_link support
On Mon, Oct 27, 2025 at 03:11:03AM +0200, Laurent Pinchart wrote:
> On Thu, Sep 25, 2025 at 12:31:56PM -0400, Frank Li wrote:
> > On Fri, Sep 12, 2025 at 12:00:41AM +0200, Marco Felsch wrote:
> > > Shift the device dependency handling to the driver core by adding
> > > support for devlinks.
> > >
> > > The link between the consumer device and the dmaengine device is
> > > established by the consumer via the dma_request_chan() automatically if
> > > the dmaengine driver supports it (create_devlink flag set).
> > >
> > > By adding the devlink support it is ensured that the supplier can't be
> > > removed while the consumer still uses the dmaengine. Furthermore it
> > > ensures that the supplier driver is present and actual bound before the
> > > consumer is uses the supplier.
>
> How is the latter ensured by this patch ? The link is created in
> dma_request_chan() (which is called by the consumer), after successfully
> obtaining the channel. I don't see how the link improves that mechanism.
>
> > >
> > > Additional PM and runtime-PM dependency handling can be added easily too
> > > by setting the required flags (not implemented by this commit).
>
> I've long thought that the DMA engine API should offer calls to "prepare"
> and "unprepare" (names subject to bikeshedding) a DMA engine channel, so
> that consumers can explicitly indicate when they are getting ready to
> use DMA, and when they stop.
This is one validate method. Maybe we set flags in dma_request_chan() to
indicate auto prepare when request chan to keep compatiblity with existed
drivers.
consumers who support prepare/unprepare can clean this flags at
dma_request_chan(). So we can smooth switch each consumer and dma-engine
driver to support this prepare/unprepare.
Frank
>
> > >
> > > The new create_devlink flag controlls the devlink creation to not cause
> > > any regressions with existing dmaengine drivers. This flag can be
> > > removed once all drivers are successfully tested to support devlinks.
> > >
> > > Signed-off-by: Marco Felsch <m.felsch@...gutronix.de>
> > > ---
> >
> > Add previous discussion link:
> > https://lore.kernel.org/all/aLhUv+mtr1uZTCth@lizhi-Precision-Tower-5810/
> >
> > Another thread
> > https://lore.kernel.org/dmaengine/20250801120007.GB4906@pendragon.ideasonboard.com/
> >
> > Add Laurent Pinchart, who may instest this topic also.
> >
> > Add Rob Herring, who may know why dma engine can't create dev_link default
> > like other provider (clk, phy, gpio ...)
> >
> >
> > > drivers/dma/dmaengine.c | 15 +++++++++++++++
> > > include/linux/dmaengine.h | 3 +++
> > > 2 files changed, 18 insertions(+)
> > >
> > > diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
> > > index 758fcd0546d8bde8e8dddc6039848feeb1e24475..e81985ab806ae87ff3aa4739fe6f6328b2587f2e 100644
> > > --- a/drivers/dma/dmaengine.c
> > > +++ b/drivers/dma/dmaengine.c
> > > @@ -858,6 +858,21 @@ struct dma_chan *dma_request_chan(struct device *dev, const char *name)
> > > /* No functional issue if it fails, users are supposed to test before use */
> > > #endif
> > >
> > > + /*
> > > + * Devlinks between the dmaengine device and the consumer device
> > > + * are optional till all dmaengine drivers are converted/tested.
> > > + */
> > > + if (chan->device->create_devlink) {
> > > + struct device_link *dl;
> > > +
> > > + dl = device_link_add(dev, chan->device->dev, DL_FLAG_AUTOREMOVE_CONSUMER);
> >
> > I suggest link to per channel device, instead dma engine devices.
> > chan->dev->device like phy drivers because some dma-engine have per channel
> > resources, like power domain and clocks.
> >
> > Frank
> >
> > > + if (!dl) {
> > > + dev_err(dev, "failed to create device link to %s\n",
> > > + dev_name(chan->device->dev));
> > > + return ERR_PTR(-EINVAL);
> > > + }
> > > + }
> > > +
> > > chan->name = kasprintf(GFP_KERNEL, "dma:%s", name);
> > > if (!chan->name)
> > > return chan;
> > > diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> > > index bb146c5ac3e4ccd7bc0afbf3b28e5b3d659ad62f..c67737a358df659f2bf050a9ccb8d23b17ceb357 100644
> > > --- a/include/linux/dmaengine.h
> > > +++ b/include/linux/dmaengine.h
> > > @@ -817,6 +817,8 @@ struct dma_filter {
> > > * DMA tansaction with no software intervention for reinitialization.
> > > * Zero value means unlimited number of entries.
> > > * @descriptor_reuse: a submitted transfer can be resubmitted after completion
> > > + * @create_devlink: create a devlink between a dma_chan_dev supplier and
> > > + * dma-channel consumer device
> > > * @residue_granularity: granularity of the transfer residue reported
> > > * by tx_status
> > > * @device_alloc_chan_resources: allocate resources and return the
> > > @@ -894,6 +896,7 @@ struct dma_device {
> > > u32 max_burst;
> > > u32 max_sg_burst;
> > > bool descriptor_reuse;
> > > + bool create_devlink;
> > > enum dma_residue_granularity residue_granularity;
> > >
> > > int (*device_alloc_chan_resources)(struct dma_chan *chan);
> > >
> > > --
> > > 2.47.3
> > >
>
> --
> Regards,
>
> Laurent Pinchart
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