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

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Peter,

On Thu, Jan 30, 2020 at 10:42 AM Peter Ujfalusi <peter.ujfalusi@ti.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@glider.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.

> > +
> > +     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.

> > +     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.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds