Message-ID: <0c2cc631-21fd-41fd-9293-fd86dd09a2d2@oss.qualcomm.com>
Date: Sat, 2 Aug 2025 14:37:54 +0200
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Frank Li <Frank.li@....com>
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Subject: Re: [PATCH RFC 2/6] dmaengine: Make of_dma_request_slave_channel pass
 a cookie to of_xlate

On 8/1/25 2:00 PM, Laurent Pinchart wrote:
> Hi Frank,
> 
> On Wed, Jul 30, 2025 at 02:37:54PM -0400, Frank Li wrote:
>> On Wed, Jul 30, 2025 at 09:04:17PM +0300, Laurent Pinchart wrote:
>>> On Wed, Jul 30, 2025 at 12:39:43PM -0400, Frank Li wrote:
>>>> On Wed, Jul 30, 2025 at 11:33:29AM +0200, Konrad Dybcio wrote:
>>>>> From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
>>>>>
>>>>> The DMA subsystem attempts to make it theoretically possible to pair
>>>>> any DMA block with any user. While that's convenient from a
>>>>> codebase sanity perspective, some blocks are more intertwined.
>>>>>
>>>>> One such case is the Qualcomm GENI, where each wrapper contains a
>>>>> number of Serial Engine instances, each one of which can be programmed
>>>>> to support a different protocol (such as I2C, I3C, SPI, UART, etc.).
>>>>>
>>>>> The GPI DMA it's designed together with, needs to receive the ID of the
>>>>> protocol that's in use, to adjust its behavior accordingly. Currently,
>>>>> that's done through passing that ID through device tree, with each
>>>>> Serial Engine expressed NUM_PROTOCOL times, resulting in terrible
>>>>> dt-bindings that are full of useless copypasta.
>>>>>
>>>>> In a step to cut down on that, let the DMA user give the engine driver
>>>>> a hint at request time.
>>>>>
>>>>> Signed-off-by: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
>>>>> ---

[...]

>>>>> diff --git a/include/linux/of_dma.h b/include/linux/of_dma.h
>>>>> index fd706cdf255c61c82ce30ef9a2c44930bef34bc8..9f9bc4207b85d48d73c25aad4b362e7c84c01756 100644
>>>>> --- a/include/linux/of_dma.h
>>>>> +++ b/include/linux/of_dma.h
>>>>> @@ -19,7 +19,7 @@ struct of_dma {
>>>>>  	struct list_head	of_dma_controllers;
>>>>>  	struct device_node	*of_node;
>>>>>  	struct dma_chan		*(*of_dma_xlate)
>>>>> -				(struct of_phandle_args *, struct of_dma *);
>>>>> +				(struct of_phandle_args *, struct of_dma *, void *);
>>>>
>>>> I suggest pass down more informaiton, like client's dev point. So we can
>>>> auto create device link between client's dev and dma chan's device.
>>>
>>> Is .of_dma_xlate() really the right place to do that ? If you want to
>>> create a device link for PM reasons, isn't it better created when the
>>> channel is requested ? It should also be removed when the channel is
>>> freed.
>>
>> I remember just need record client device pointer here.
>>
>>>>
>>>> DMA Engineer device
>>>>    DMA chan device
>>>>        consumer clients' device.
>>>>
>>>> If consumer device runtime pm suspend can auto trigger DMA chan's device's
>>>> runtime pm function.
>>>>
>>>> It will simplifly DMA engine's run time pm manage. Currently many DMA run
>>>> time pm implement as, runtime_pm_get() when alloc and runtime_pm_put() at
>>>> free channel.  But many devices request dma channel at probe, which make
>>>> dma engine work at always 'on' state.
>>>>
>>>> But ideally, dma chan should be resume only when it is used to transfer.
>>>
>>> This is exactly what I was going to mention after reading the last
>>> paragraph. Is there anything that prevents a DMA engine driver to
>>> perform a rutime PM get() when a transfer is submitted
>>
>> DMA description is a queue, It is hard to track each descriptor submit and
>> finished. espcially cycle buffer case.
>>
>> And according to dma engine API defination, submit a descriptor not
>> neccessary to turn on clock, maybe just pure software operation, such as
>> enqueue it to a software list.
>>
>> Many driver call dmaengine_submit() in irq context,  submit new descriptor
>> when previous descriptor finished. runtime_pm_get() can NOT be called in
>> atomic context.
>>
>> And some driver submit many descripor advance. Only issue_transfer() is
>> actually trigger hardware to start transfer.
>>
>> Some client use cycle descripor, such audio devices.  Some audio devices
>> have not free descriptor at their run time suspend function, just disable
>> audio devices's clocks.  Audio devices run time suspend, which means no
>> one use this dma channel, dma channel can auto suspend if built device link
>> between audio device and dma chan devices.
>>
>> Some DMA client have not devices, such as memory to memory. for this kind
>> case, it need keep chan always on.
>>
>> issue_transfer() can be call in atomic context. but trigger hardware transfer
>> need clock and runtime_pm_get() can't be called in atomic context.
>>
>> Most case issue_transfer() is call in irq handle, which means device should
>> already be in runtime resume statue.  DMA engine can safely access their
>> register if using device link.
> 
> You have good points there, in particular the fact the issue_transfer()
> can be called in interrupt context.
> 
> For me this calls for new DMA engine operations to "start/stop" the DMA
> engine (better names are likely needed) from a client perspective.
> 
>>> and a put() when
>>> it completes ? (Logically speaking, the actual implementation would
>>> likely be a bit different in drivers, but the result would be similar.)

So.. do you folks want me to alter the patch in any way?

Konrad

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