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SubjectRe: [PATCH v4 02/10] dmaengine: Actions: Add support for S700 DMA engine
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On 29/06/2020 10:54, Vinod Koul wrote:

Hi Vinod,

> On 24-06-20, 10:35, Andr� Przywara wrote:
>> On 24/06/2020 07:15, Vinod Koul wrote:
>>> On 09-06-20, 15:47, Amit Singh Tomar wrote:
>>>
>>>> @@ -372,6 +383,7 @@ static inline int owl_dma_cfg_lli(struct owl_dma_vchan *vchan,
>>>> struct dma_slave_config *sconfig,
>>>> bool is_cyclic)
>>>> {
>>>> + struct owl_dma *od = to_owl_dma(vchan->vc.chan.device);
>>>> u32 mode, ctrlb;
>>>>
>>>> mode = OWL_DMA_MODE_PW(0);
>>>> @@ -427,14 +439,26 @@ static inline int owl_dma_cfg_lli(struct owl_dma_vchan *vchan,
>>>> lli->hw[OWL_DMADESC_DADDR] = dst;
>>>> lli->hw[OWL_DMADESC_SRC_STRIDE] = 0;
>>>> lli->hw[OWL_DMADESC_DST_STRIDE] = 0;
>>>> - /*
>>>> - * Word starts from offset 0xC is shared between frame length
>>>> - * (max frame length is 1MB) and frame count, where first 20
>>>> - * bits are for frame length and rest of 12 bits are for frame
>>>> - * count.
>>>> - */
>>>> - lli->hw[OWL_DMADESC_FLEN] = len | FCNT_VAL << 20;
>>>> - lli->hw[OWL_DMADESC_CTRLB] = ctrlb;
>>>> +
>>>> + if (od->devid == S700_DMA) {
>>>> + /* Max frame length is 1MB */
>>>> + lli->hw[OWL_DMADESC_FLEN] = len;
>>>> + /*
>>>> + * On S700, word starts from offset 0x1C is shared between
>>>> + * frame count and ctrlb, where first 12 bits are for frame
>>>> + * count and rest of 20 bits are for ctrlb.
>>>> + */
>>>> + lli->hw[OWL_DMADESC_CTRLB] = FCNT_VAL | ctrlb;
>>>> + } else {
>>>> + /*
>>>> + * On S900, word starts from offset 0xC is shared between
>>>> + * frame length (max frame length is 1MB) and frame count,
>>>> + * where first 20 bits are for frame length and rest of
>>>> + * 12 bits are for frame count.
>>>> + */
>>>> + lli->hw[OWL_DMADESC_FLEN] = len | FCNT_VAL << 20;
>>>> + lli->hw[OWL_DMADESC_CTRLB] = ctrlb;
>>>
>>> Unfortunately this wont scale, we will keep adding new conditions for
>>> newer SoC's! So rather than this why not encode max frame length in
>>> driver_data rather than S900_DMA/S700_DMA.. In future one can add values
>>> for newer SoC and not code above logic again.
>>
>> What newer SoCs? I don't think we should try to guess the future here.
>
> In a patch for adding new SoC, quite ironical I would say!

S700 is not a new SoC, it's just this driver didn't support it yet. What
I meant is that I don't even know about the existence of upcoming SoCs
(Google seems clueless), not to speak of documentation to assess which
DMA controller they use.

>> We can always introduce further abstractions later, once we actually
>> *know* what we are looking at.
>
> Rather if we know we are adding abstractions, why not add in a way that
> makes it scale better rather than rework again

I appreciate the effort, but this really tapping around in the dark,
since we don't know which direction any new DMA controller is taking. I
might not even be similar.

>> Besides, I don't understand what you are after. The max frame length is
>> 1MB in both cases, it's just a matter of where to put FCNT_VAL, either
>> in FLEN or in CTRLB. And having an extra flag for that in driver data
>> sounds a bit over the top at the moment.
>
> Maybe, maybe not. I would rather make it support N SoC when adding
> support for second one rather than keep adding everytime a new SoC is
> added...

Well, what do you suggest, specifically? At the moment we have two
*slightly* different DMA controllers, so we differentiate between the
two based on the model. Do you want to introduce an extra flag like
FRAME_CNT_IN_CTRLB? That seems to be a bit over the top here, since we
don't know if a future DMA controller is still compatible, or introduces
completely new differences.

Cheers,
Andre

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