Re: [alsa-devel] [PATCH v5 06/10] dmaengine: add API for getting dma controller's quirk

[Lars-Peter Clausen]

On 10/09/2015 01:31 PM, Shawn Lin wrote:
> å 2015/10/8 16:31, Lars-Peter Clausen åé:
>> On 10/06/2015 11:21 AM, Shawn Lin wrote:
>>> Hi Vinod,
>>>
>>> On 2015/10/5 23:37, Vinod Koul wrote:
>>>> On Mon, Sep 14, 2015 at 07:48:59AM +0800, Shawn Lin wrote:
>>>>> Add dmaengine_get_quirks API for peripheral devices to query
>>>>> quirks if they need it to make special workaround due to broken
>>>>> dma controller design.
>>>>>
>>>>> Signed-off-by: Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>
>>>>> ---
>>>>>
>>>>> Changes in v5: None
>>>>> Changes in v4: None
>>>>> Changes in v3: None
>>>>> Changes in v2: None
>>>>> Changes in v1: None
>>>>>
>>>>>    include/linux/dmaengine.h | 9 +++++++++
>>>>>    1 file changed, 9 insertions(+)
>>>>>
>>>>> diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
>>>>> index e2f5eb4..5174ca4 100644
>>>>> --- a/include/linux/dmaengine.h
>>>>> +++ b/include/linux/dmaengine.h
>>>>> @@ -704,6 +704,7 @@ struct dma_device {
>>>>>
>>>>>        int (*device_config)(struct dma_chan *chan,
>>>>>                     struct dma_slave_config *config);
>>>>> +    int (*device_get_quirks)(struct dma_chan *chan);
>>>>
>>>> And why do we want to expose this to users? THis doesnt seem right!
>>>>
>>>
>>> Basically I agree not to expose dma's quirk to slave controllers...But, the
>>> fact I mentioned on cover letter explain the reasons why I have to let slave
>>> controllers know that they are working with a broken dma. It's a dilemma
>>> that if we don't want that to be exposed(let slave controllers' driver get
>>> the info via a API), we have t add broken quirk for all of them ,here and
>>> there, which seems to be a disaster:(
>>
>> The problem with this API is that it transports values with device specific
>> meanings over a generic API. Which is generally speaking not a good idea
>> because the consumer witch is supposed to be generic suddenly needs to know
>> which provider it is talking to.
>>
>> A better solution in this case typically is either introduce a generic API
>> with generic values or a custom API with custom values, but don't mix the
>> two.
>>
>>>
>>> I would appreciate it if you could give me some suggestions at your earliest
>>> convenience. :)
>>
>> In this case I think the best way to handle this is not quirks, but rather
>> expose the actual maximum burst size using the DMA capabilities API. Since
>> supporting only a certain burst depth is not really a quirk. All hardware
>> has a limit for this and for some it might be larger or smaller than for
>> others and it might be the same IP core but the maximum size depends on some
>> IP core parameters. So this should be discoverable.
>>
> 
> Hi Lars,
> 
> Thanks for looking for that.
> 
> It's a good idea if all clients of the Soc are broken, but unfortunately
> some of them work. So... max burst shoule be different for individuals.

Well, the dma_get_slave_caps() API works on a DMA channel, so I don't think
this will be a problem.

- Lars


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