linux-kernel - Re: [PATCH/RFC] dmaengine: Create symlinks from DMA channels to slaves

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Date:   Mon, 10 Jun 2019 13:24:27 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Geert Uytterhoeven <geert+renesas@...der.be>
Cc:     Dan Williams <dan.j.williams@...el.com>, dmaengine@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH/RFC] dmaengine: Create symlinks from DMA channels to
 slaves

On 07-06-19, 13:38, Geert Uytterhoeven wrote:
> Currently it is not easy to find out which DMA channels are in use, and
> by which slave devices.
> 
> Fix this by creating in sysfs a "slave" symlink from the DMA channel to
> the actual slave device when a channel is requested, and removing it
> again when the channel is released.
> 
> For now this is limited to DT and ACPI.
> 
> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
> ---
> Questions:
>   1. Do you think this is useful?

Yes for me at least :)

>   2. Should backlinks (e.g. "dma:<name>") be created from the slave
>      device to the DMA channel?
>      This requires storing the name in struct dma_chan, for later
>      symlink removal.

that would certainly be more helpful

>   3. Should this be extended to other ways of requesting channels?
>      In many cases, no device pointer is available, so a device pointer
>      parameter has to be added to all DMA channel request APIs that
>      don't have it yet.

I think that would need to be done.

> ---
>  drivers/dma/dmaengine.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
> index 03ac4b96117cd8db..c11476f76fc96bcf 100644
> --- a/drivers/dma/dmaengine.c
> +++ b/drivers/dma/dmaengine.c
> @@ -706,6 +706,10 @@ struct dma_chan *dma_request_chan(struct device *dev, const char *name)
>  
>  	if (chan) {
>  		/* Valid channel found or requester needs to be deferred */
> +		if (!IS_ERR(chan) &&
> +		     sysfs_create_link(&chan->dev->device.kobj, &dev->kobj,
> +				       "slave"))
> +			dev_err(dev, "Cannot create DMA slave symlink\n");
>  		if (!IS_ERR(chan) || PTR_ERR(chan) == -EPROBE_DEFER)
>  			return chan;
>  	}
> @@ -786,6 +790,7 @@ void dma_release_channel(struct dma_chan *chan)
>  	/* drop PRIVATE cap enabled by __dma_request_channel() */
>  	if (--chan->device->privatecnt == 0)
>  		dma_cap_clear(DMA_PRIVATE, chan->device->cap_mask);
> +	sysfs_remove_link(&chan->dev->device.kobj, "slave");
>  	mutex_unlock(&dma_list_mutex);
>  }
>  EXPORT_SYMBOL_GPL(dma_release_channel);
> -- 
> 2.17.1

-- 
~Vinod