Re: [PATCH v8 1/3] dmaengine: Add new device_{set,release}_slave callbacks

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hi Vinod,

On 2017-02-10 05:34, Vinod Koul wrote:
> On Thu, Feb 09, 2017 at 03:22:49PM +0100, Marek Szyprowski wrote:
>> Add two new callbacks to DMA engine device. They will used to provide
>> access to slave device (the device which requested given DMA channel)
> You mean access to client devices?

Yes. It looks that I was confused by the code, where the term 'slave'
appears a few times. 'Client' is a bit more appropriate then.

>> for DMA engine driver. Access to slave device might be useful for example
>> for implementing advanced runtime power management.
>>
>> DMA slave channels are exclusive, so only one slave device can be set
>> for a given DMA slave channel.
> That is not a right assumption and my worry here. With virt-dma we don't
> really assume a hardware channel and exclusive. Certain implementation may
> do that but from framework we cannot assume that.

Okay, I came to such conclusion basing one the dma engine code, but maybe
I missed something. However in such case such callback will be called for
each client device and it will be up to the driver to handle that.

>> device_set_slave() will be called after the device_alloc_chan_resources()
>> and device_release_slave() before the device_free_chan_resources().
> Okay, I had to relook at the series to get around this part. Sorry but we
> can't call it set_slave, it is actually set_client/consumer

That's okay, the name of the callbacks should be changed.

> In our context slaves means dmaengine slave devices aka provider.
> Client would be the consumer and not slave.

I'm a new to the DMA engine framework, I'm sorry for using wrong terms.

>> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
>> ---
>>   drivers/dma/dmaengine.c   | 27 ++++++++++++++++++++++++---
>>   include/linux/dmaengine.h | 10 ++++++++++
>>   2 files changed, 34 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
>> index 24e0221fd66d..5b7089d8be4d 100644
>> --- a/drivers/dma/dmaengine.c
>> +++ b/drivers/dma/dmaengine.c
>> @@ -705,6 +705,7 @@ struct dma_chan *dma_request_chan(struct device *dev, const char *name)
>>   {
>>   	struct dma_device *d, *_d;
>>   	struct dma_chan *chan = NULL;
>> +	int ret;
>>   
>>   	/* If device-tree is present get slave info from here */
>>   	if (dev->of_node)
>> @@ -715,8 +716,9 @@ struct dma_chan *dma_request_chan(struct device *dev, const char *name)
>>   		chan = acpi_dma_request_slave_chan_by_name(dev, name);
>>   
>>   	if (chan) {
>> -		/* Valid channel found or requester need to be deferred */
>> -		if (!IS_ERR(chan) || PTR_ERR(chan) == -EPROBE_DEFER)
>> +		if (!IS_ERR(chan))
>> +			goto found;
>> +		if (PTR_ERR(chan) == -EPROBE_DEFER)
>>   			return chan;
>>   	}
>>   
>> @@ -738,7 +740,21 @@ 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 (!chan)
>> +		return ERR_PTR(-EPROBE_DEFER);
>> +	if (IS_ERR(chan))
>> +		return chan;
>> +found:
>> +	if (chan->device->device_set_slave) {
>> +		chan->slave = dev;
>> +		ret = chan->device->device_set_slave(chan, dev);
>> +		if (ret) {
>> +			chan->slave = NULL;
>> +			dma_release_channel(chan);
>> +			chan = ERR_PTR(ret);
>> +		}
>> +	}
>> +	return chan;
>>   }
>>   EXPORT_SYMBOL_GPL(dma_request_chan);
>>   
>> @@ -786,6 +802,11 @@ void dma_release_channel(struct dma_chan *chan)
>>   	mutex_lock(&dma_list_mutex);
>>   	WARN_ONCE(chan->client_count != 1,
>>   		  "chan reference count %d != 1\n", chan->client_count);
>> +	if (chan->slave) {
>> +		if (chan->device->device_release_slave)
>> +			chan->device->device_release_slave(chan);
>> +		chan->slave = NULL;
>> +	}
>>   	dma_chan_put(chan);
>>   	/* drop PRIVATE cap enabled by __dma_request_channel() */
>>   	if (--chan->device->privatecnt == 0)
>> diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
>> index 533680860865..d22299e37e69 100644
>> --- a/include/linux/dmaengine.h
>> +++ b/include/linux/dmaengine.h
>> @@ -277,6 +277,9 @@ struct dma_chan {
>>   	struct dma_router *router;
>>   	void *route_data;
>>   
>> +	/* Only for SLAVE channels */
>> +	struct device *slave;
> so assuming you refer to consumer aka client here, why do we need set if we
> store it here.

DMA engine driver might need to do something with it (like setting up a pm
link for example) before starting any operations. It would be great if the
pointer to client device is available in device_alloc_chan_resources(), but
propagating it there is not possible without significant changes. That's why
I came with this a separate callback.

Maybe the client device shouldn't be stored in the dma_chan structure at all
and left to the drivers to use or manage it if really needed. This will also
solve the issue with virt-dma you have mentioned.

In the previous version I managed to pass client device pointer to
device_alloc_chan_resources() via of_xlate callback (please take a look into
v7), but that approach was rejected by Lars-Peter Clausen.

 > ...

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