linux-kernel - Re: [PATCH v2] dmaengine: Create symlinks between DMA channels and slaves

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Date:   Wed, 29 Jan 2020 18:47:21 +0100
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Vinod Koul <vkoul@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     dmaengine@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] dmaengine: Create symlinks between DMA channels and
 slaves

Hi Geert,

On 17.01.2020 16:30, Geert Uytterhoeven wrote:
> Currently it is not easy to find out which DMA channels are in use, and
> which slave devices are using which channels.
>
> Fix this by creating two symlinks between the DMA channel and the actual
> slave device when a channel is requested:
>    1. A "slave" symlink from DMA channel to slave device,
>    2. A "dma:<name>" symlink slave device to DMA channel.
> When the channel is released, the symlinks are removed again.
> The latter requires keeping track of the slave device and the channel
> name in the dma_chan structure.
>
> Note that this is limited to channel request functions for requesting an
> exclusive slave channel that take a device pointer (dma_request_chan()
> and dma_request_slave_channel*()).
>
> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
> Tested-by: Niklas Söderlund <niklas.soderlund@...natech.se>

This patch breaks booting on almost all Exynos based boards:

https://lore.kernel.org/linux-samsung-soc/20200129161113.GE3928@sirena.org.uk/T/#u

I've already sent a fix:

https://lkml.org/lkml/2020/1/29/498

BTW, this patch reminds me some of my earlier work:

https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1329778.html

I had similar need to keep a client's struct device pointer for every 
requested channel, but it turned out to be much more complicated than 
I've initially thought. I've abandoned that, due to lack of time, but 
maybe some of that discussion and concerns are still valid (I hope that 
links to earlier versions are still working)...

 > ...

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland