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

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Message-ID: <bdba1ecd-b47a-f790-7385-cfad364423df@ti.com>
Date:   Thu, 30 Jan 2020 12:22:37 +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

Hi Geert,

On 30/01/2020 11.51, Geert Uytterhoeven wrote:
> Hi Peter,
> 
> On Thu, Jan 30, 2020 at 10:42 AM Peter Ujfalusi <peter.ujfalusi@...com> wrote:
>> On 17/01/2020 17.30, 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>
> 
>>> --- a/drivers/dma/dmaengine.c
>>> +++ b/drivers/dma/dmaengine.c
>>> @@ -60,6 +60,8 @@ static long dmaengine_ref_count;
>>>
>>>  /* --- sysfs implementation --- */
>>>
>>> +#define DMA_SLAVE_NAME       "slave"
>>> +
>>>  /**
>>>   * dev_to_dma_chan - convert a device pointer to its sysfs container object
>>>   * @dev - device node
>>> @@ -730,11 +732,11 @@ struct dma_chan *dma_request_chan(struct device *dev, const char *name)
>>>       if (has_acpi_companion(dev) && !chan)
>>>               chan = acpi_dma_request_slave_chan_by_name(dev, name);
>>>
>>> -     if (chan) {
>>> -             /* Valid channel found or requester needs to be deferred */
>>> -             if (!IS_ERR(chan) || PTR_ERR(chan) == -EPROBE_DEFER)
>>> -                     return chan;
>>> -     }
>>> +     if (PTR_ERR(chan) == -EPROBE_DEFER)
>>> +             return chan;
>>> +
>>> +     if (!IS_ERR_OR_NULL(chan))
>>> +             goto found;
>>>
>>>       /* Try to find the channel via the DMA filter map(s) */
>>>       mutex_lock(&dma_list_mutex);
>>> @@ -754,7 +756,23 @@ struct dma_chan *dma_request_chan(struct device *dev, const char *name)
>>>       }
>>>       mutex_unlock(&dma_list_mutex);
>>>
>>> -     return chan ? chan : ERR_PTR(-EPROBE_DEFER);
>>> +     if (!IS_ERR_OR_NULL(chan))
>>> +             goto found;
>>> +
>>> +     return ERR_PTR(-EPROBE_DEFER);
>>> +
>>> +found:
>>> +     chan->slave = dev;
>>> +     chan->name = kasprintf(GFP_KERNEL, "dma:%s", name);
>>> +     if (!chan->name)
>>> +             return ERR_PTR(-ENOMEM);
>>
>> You will lock the channel... It is requested, but it is not released in
>> case of failure.
> 
> True. Perhaps this error should just be ignored, cfr. below.
> However, if this operation fails, chances are high the system will die very soon
> anyway.

Yeah, I'll fix it up in a series I'm preparing.

> 
>>> +
>>> +     if (sysfs_create_link(&chan->dev->device.kobj, &dev->kobj,
>>> +                           DMA_SLAVE_NAME))
>>> +             dev_err(dev, "Cannot create DMA %s symlink\n", DMA_SLAVE_NAME);
>>> +     if (sysfs_create_link(&dev->kobj, &chan->dev->device.kobj, chan->name))
>>> +             dev_err(dev, "Cannot create DMA %s symlink\n", chan->name);
>>
>> It is not a problem if these fail?
> 
> IMHO, a failure to create these links is not fatal for the operation of
> the device, and thus can be ignored.  Just like for debugfs.

OK, then these should not be dev_err, but dev_warn.
I'll include this is also in a fixup patch.

> 
>>> +     return chan;
>>>  }
>>>  EXPORT_SYMBOL_GPL(dma_request_chan);
>>>
>>> @@ -812,6 +830,13 @@ 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);
>>> +     if (chan->slave) {
>>> +             sysfs_remove_link(&chan->slave->kobj, chan->name);
>>> +             kfree(chan->name);
>>> +             chan->name = NULL;
>>> +             chan->slave = NULL;
>>> +     }
>>> +     sysfs_remove_link(&chan->dev->device.kobj, DMA_SLAVE_NAME);
>>
>> If a non slave channel is released, then you remove the link you have
>> never created?
>>
>> What happens if the link creation fails and here you attempt to remove
>> the failed ones?
> 
> sysfs_remove_link() should handle removing non-existent links, and just
> return -ENOENT.

True, just followed the call chain and tested as well, but the
DMA_SLAVE_NAME symlink should be also removed within the
if (chan->slave) {} block as it is never created for non slave channels.

Also including inn my fixup patch.

> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 

- Péter

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