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

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Date:	Thu, 08 Oct 2015 10:31:18 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Shawn Lin <shawn.lin@...k-chips.com>,
	Vinod Koul <vinod.koul@...el.com>
CC:	Addy Ke <addy.ke@...k-chips.com>, Heiko Stuebner <heiko@...ech.de>,
	alsa-devel@...a-project.org, linux-rockchip@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org,
	Doug Anderson <dianders@...omium.org>,
	Takashi Iwai <tiwai@...e.com>, dmaengine@...r.kernel.org,
	Mark Brown <broonie@...nel.org>,
	Olof Johansson <olof@...om.net>,
	Sonny Rao <sonnyrao@...omium.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [alsa-devel] [PATCH v5 06/10] dmaengine: add API for getting
 dma controller's quirk

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@...k-chips.com>
>>> ---
>>>
>>> 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.

- Lars

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