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Subject: Re: [PATCH 2/2] dmaengine: Add basic debugfs support
To:     Geert Uytterhoeven <geert@linux-m68k.org>
CC:     Vinod <vkoul@kernel.org>, <dmaengine@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Dan Williams <dan.j.williams@intel.com>
References: <20200130114220.23538-1-peter.ujfalusi@ti.com>
 <20200130114220.23538-3-peter.ujfalusi@ti.com>
 <CAMuHMdWXuoWX=AgB=RY=5At_yw6nZJGBOK_8TXjwgYQN27JcQQ@mail.gmail.com>
From:   Peter Ujfalusi <peter.ujfalusi@ti.com>
Message-ID: <aa574e2e-222c-de76-dbc5-b117190648d6@ti.com>
Date:   Fri, 31 Jan 2020 10:29:22 +0200
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Hi Geert,

On 30/01/2020 17.42, Geert Uytterhoeven wrote:
> Hi Peter,
> 
> On Thu, Jan 30, 2020 at 12:41 PM Peter Ujfalusi <peter.ujfalusi@ti.com> wrote:
>> Via the /sys/kernel/debug/dmaengine users can get information about the
>> DMA devices and the used channels.
>>
>> Example output on am654-evm with audio using two channels and after running
>> dmatest on 6 channels:
>>
>>  # cat /sys/kernel/debug/dmaengine
>> dma0 (285c0000.dma-controller): number of channels: 96
>>
>> dma1 (31150000.dma-controller): number of channels: 267
>>  dma1chan0:             2b00000.mcasp:tx
>>  dma1chan1:             2b00000.mcasp:rx
>>  dma1chan2:             in-use
>>  dma1chan3:             in-use
>>  dma1chan4:             in-use
>>  dma1chan5:             in-use
>>  dma1chan6:             in-use
>>  dma1chan7:             in-use
>>
>> For slave channels we can show the device and the channel name a given
>> channel is requested.
>> For non slave devices the only information we know is that the channel is
>> in use.
>>
>> DMA drivers can implement the optional dbg_show callback to provide
>> controller specific information instead of the generic one.
>>
>> It is easy to extend the generic dmaengine_dbg_show() to print additional
>> information about the used channels.
>>
>> I have taken the idea from gpiolib.
>>
>> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
> 
> Thanks for your patch!
> 
> On Salvator-XS with R-Car H3 ES2.0:
> 
>     dma0 (ec700000.dma-controller): number of channels: 15
> 
>     dma1 (ec720000.dma-controller): number of channels: 15
> 
>     dma2 (e65a0000.dma-controller): number of channels: 2
>      dma2chan0: e6590000.usb:ch0
>      dma2chan1: e6590000.usb:ch1
> 
>     dma3 (e65b0000.dma-controller): number of channels: 2
>      dma3chan0: e6590000.usb:ch2
>      dma3chan1: e6590000.usb:ch3
> 
>     dma4 (e6460000.dma-controller): number of channels: 2
>      dma4chan0: e659c000.usb:ch0
>      dma4chan1: e659c000.usb:ch1
> 
>     dma5 (e6470000.dma-controller): number of channels: 2
>      dma5chan0: e659c000.usb:ch2
>      dma5chan1: e659c000.usb:ch3
> 
>     dma6 (e6700000.dma-controller): number of channels: 15
> 
>     dma7 (e7300000.dma-controller): number of channels: 15
>      dma7chan0: e6510000.i2c:tx
> 
>     dma8 (e7310000.dma-controller): number of channels: 15
>      dma8chan0: e6550000.serial:tx
>      dma8chan1: e6550000.serial:rx

You have lots of DMAs over there ;)

>> --- a/drivers/dma/dmaengine.c
>> +++ b/drivers/dma/dmaengine.c
>> @@ -760,6 +761,13 @@ struct dma_chan *dma_request_chan(struct device *dev, const char *name)
>>                 return chan ? chan : ERR_PTR(-EPROBE_DEFER);
>>
>>  found:
>> +#ifdef CONFIG_DEBUG_FS
>> +       chan->slave_name = kasprintf(GFP_KERNEL, "%s:%s", dev_name(dev), name);
>> +       if (!chan->slave_name)
>> +               dev_warn(dev,
>> +                        "Cannot allocate memory for slave name (debugfs)\n");
> 
> No need to print a message, as the memory allocation core already takes
> care of that.

Right.

> But, do you really need chan->slave_name?
> You already have chan->slave and chan->name.

The chan->name is prefixed with "dma:" it would not look right.
In production this all go away as debugfs most likely disabled.
But I will change the name to dbg_client_name.

> 
>> +#endif
>> +
>>         chan->name = kasprintf(GFP_KERNEL, "dma:%s", name);
>>         if (!chan->name) {
>>                 dev_warn(dev,
> 
>> @@ -1562,3 +1577,108 @@ static int __init dma_bus_init(void)
>>         return class_register(&dma_devclass);
>>  }
>>  arch_initcall(dma_bus_init);
>> +
>> +#ifdef CONFIG_DEBUG_FS
>> +static void *dmaengine_seq_start(struct seq_file *s, loff_t *pos)
>> +{
>> +       struct dma_device *dma_dev = NULL;
>> +       loff_t index = *pos;
>> +
>> +       s->private = "";
>> +
>> +       mutex_lock(&dma_list_mutex);
>> +       list_for_each_entry(dma_dev, &dma_device_list, global_node)
>> +               if (index-- == 0) {
>> +                       mutex_unlock(&dma_list_mutex);
>> +                       return dma_dev;
> 
> Can the dma_device go away after unlocking the list?
> Unlike dma_request_chan(), this doesn't increase a refcnt.

It could, let me see what I can do. Probably locking the dma_device_list
for the duration of the show.

>> +               }
>> +       mutex_unlock(&dma_list_mutex);
>> +
>> +       return NULL;
>> +}
>> +
>> +static void *dmaengine_seq_next(struct seq_file *s, void *v, loff_t *pos)
>> +{
>> +       struct dma_device *dma_dev = v;
>> +       void *ret = NULL;
>> +
>> +       mutex_lock(&dma_list_mutex);
>> +       if (list_is_last(&dma_dev->global_node, &dma_device_list))
>> +               ret = NULL;
>> +       else
>> +               ret = list_entry(dma_dev->global_node.next,
>> +                                struct dma_device, global_node);
>> +       mutex_unlock(&dma_list_mutex);
> 
> Likewise.
> 
>> +
>> +       s->private = "\n";
>> +       ++*pos;
>> +
>> +       return ret;
>> +}
>> +
>> +static void dmaengine_seq_stop(struct seq_file *s, void *v)
>> +{
>> +}
>> +
>> +static void dmaengine_dbg_show(struct seq_file *s, struct dma_device *dma_dev)
>> +{
>> +       struct dma_chan *chan;
>> +
>> +       list_for_each_entry(chan, &dma_dev->channels, device_node) {
>> +               if (chan->client_count) {
>> +                       seq_printf(s, " dma%dchan%d:", dma_dev->dev_id,
>> +                                  chan->chan_id);
>> +                       if (chan->slave_name)
>> +                               seq_printf(s, "\t\t%s\n", chan->slave_name);
>> +                       else
>> +                               seq_printf(s, "\t\t%s\n", "in-use");
> 
> The truncated ternary operator might help here:
> 
>         seq_printf(s, "\t\t%s\n", chan->slave_name ?: "in-use");
> 
> However, you might as well just use dev_name(chan->slave) and chan->name
> instead of chan->slave_name.

"2b00000.mcasp" + "dma:tx" would be an awkward combination ;)

> 
>> +               }
>> +       }
>> +}
>> +
>> +static int dmaengine_seq_show(struct seq_file *s, void *v)
>> +{
>> +       struct dma_device *dma_dev = v;
>> +
>> +       seq_printf(s, "%sdma%d (%s): number of channels: %u\n",
>> +                  (char *)s->private, dma_dev->dev_id, dev_name(dma_dev->dev),
>> +                  dma_dev->chancnt);
>> +
>> +       if (dma_dev->dbg_show)
>> +               dma_dev->dbg_show(s, dma_dev);
> 
> So providing a custom .dbg_show() means replacing the standard info, not
> augmenting it?

Correct, if a DMA driver decides to implement it, then it is it's
responsibility to show things after the
"dma%d (%s): number of channels: %u\n" line.

The standard infor is pretty minimal and not sure if it can be more verbose.
Oh, I can add the router information if it is used.

> 
>> +       else
>> +               dmaengine_dbg_show(s, dma_dev);
>> +
>> +       return 0;
>> +}
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 

- Péter

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