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

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Date:   Mon, 20 Jan 2020 12:16:33 +0200
From:   Peter Ujfalusi <peter.ujfalusi@...com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
CC:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Vinod Koul <vkoul@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        <dmaengine@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] dmaengine: Create symlinks between DMA channels and
 slaves



On 20/01/2020 11.01, Geert Uytterhoeven wrote:
> Hi Peter,
> 
> On Fri, Jan 17, 2020 at 9:08 PM Peter Ujfalusi <peter.ujfalusi@...com> wrote:
>> On 1/17/20 5:30 PM, 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,
>>
>> Have you considered similar link name as on the slave device:
>> slave:<name>
>>
>> That way it would be easier to grasp which channel is used for what
>> purpose by only looking under /sys/class/dma/ and no need to check the
>> slave device.
> 
> Would this really provide more information?
> The device name is already provided in the target of the symlink:
> 
> root@...lsch:~# readlink
> /sys/devices/platform/soc/e6720000.dma-controller/dma/dma1chan2/slave
> ../../../ee140000.sd

e6720000.dma-controller/dma/dma1chan2/slave -> ../../../ee140000.sd
e6720000.dma-controller/dma/dma1chan3/slave -> ../../../ee140000.sd

It is hard to tell which one is the tx and RX channel without looking
under the ee140000.sd:

ee140000.sd/dma:rx -> ../e6720000.dma-controller/dma/dma1chan3
ee140000.sd/dma:tx -> ../e6720000.dma-controller/dma/dma1chan2

Another option would be to not have symlinks, but a debugfs file where
this information can be extracted and would only compiled if debugfs is
enabled.

>>>   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>
>>> ---
>>> v2:
>>>   - Add DMA_SLAVE_NAME macro,
>>>   - Also handle channels from FIXME,
>>>   - Add backlinks from slave device to DMA channel,
>>>
>>> On r8a7791/koelsch, the following new symlinks are created:
>>>
>>>     /sys/devices/platform/soc/
>>>     ├── e6700000.dma-controller/dma/dma0chan0/slave -> ../../../e6e20000.spi
>>>     ├── e6700000.dma-controller/dma/dma0chan1/slave -> ../../../e6e20000.spi
>>>     ├── e6700000.dma-controller/dma/dma0chan2/slave -> ../../../ee100000.sd
>>>     ├── e6700000.dma-controller/dma/dma0chan3/slave -> ../../../ee100000.sd
>>>     ├── e6700000.dma-controller/dma/dma0chan4/slave -> ../../../ee160000.sd
>>>     ├── e6700000.dma-controller/dma/dma0chan5/slave -> ../../../ee160000.sd
>>>     ├── e6700000.dma-controller/dma/dma0chan6/slave -> ../../../e6e68000.serial
>>>     ├── e6700000.dma-controller/dma/dma0chan7/slave -> ../../../e6e68000.serial
>>>     ├── e6720000.dma-controller/dma/dma1chan0/slave -> ../../../e6b10000.spi
>>>     ├── e6720000.dma-controller/dma/dma1chan1/slave -> ../../../e6b10000.spi
>>>     ├── e6720000.dma-controller/dma/dma1chan2/slave -> ../../../ee140000.sd
>>>     ├── e6720000.dma-controller/dma/dma1chan3/slave -> ../../../ee140000.sd
>>>     ├── e6b10000.spi/dma:rx -> ../e6720000.dma-controller/dma/dma1chan1
>>>     ├── e6b10000.spi/dma:tx -> ../e6720000.dma-controller/dma/dma1chan0
>>>     ├── e6e20000.spi/dma:rx -> ../e6700000.dma-controller/dma/dma0chan1
>>>     ├── e6e20000.spi/dma:tx -> ../e6700000.dma-controller/dma/dma0chan0
>>>     ├── e6e68000.serial/dma:rx -> ../e6700000.dma-controller/dma/dma0chan7
>>>     ├── e6e68000.serial/dma:tx -> ../e6700000.dma-controller/dma/dma0chan6
>>>     ├── ee100000.sd/dma:rx -> ../e6700000.dma-controller/dma/dma0chan3
>>>     ├── ee100000.sd/dma:tx -> ../e6700000.dma-controller/dma/dma0chan2
>>>     ├── ee140000.sd/dma:rx -> ../e6720000.dma-controller/dma/dma1chan3
>>>     ├── ee140000.sd/dma:tx -> ../e6720000.dma-controller/dma/dma1chan2
>>>     ├── ee160000.sd/dma:rx -> ../e6700000.dma-controller/dma/dma0chan5
>>>     └── ee160000.sd/dma:tx -> ../e6700000.dma-controller/dma/dma0chan4
> 
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 

- Péter

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