Re: [PATCH V4 2/3] dmaengine: tegra-adma: Add support for Tegra210 ADMA

[Jon Hunter <jonathanh-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>]

On 13/04/16 14:49, Vinod Koul wrote:
> On Tue, Apr 12, 2016 at 05:23:32PM +0100, Jon Hunter wrote:
>>>>> why should this be hard coded in kernel and not queried from something like
>>>>> DT? This case seems to be hardware property
>>>>
>>>> Originally, I did have this in DT, however, the Tegra maintainers prefer
>>>> this and this is consistent with the other Tegra DMA driver (see
>>>> driver/dma/tegra20-apb-dma.c) [0].
>>>
>>> But this creates a problem when you have next generation of controller with
>>> different channel count!
>>> How do we solve then?
>>
>> Same way we solve this for the tegra20-apb-dma driver by having
>> different chip data per SoC in the driver ...
>>
>> 1259 /* Tegra20 specific DMA controller information */
>> 1260 static const struct tegra_dma_chip_data tegra20_dma_chip_data = {
>> 1261         .nr_channels            = 16,
>> 1262         .channel_reg_size       = 0x20,
>> 1263         .max_dma_count          = 1024UL * 64,
>> 1264         .support_channel_pause  = false,
>> 1265         .support_separate_wcount_reg = false,
>> 1266 };
>> 1267
>> 1268 /* Tegra30 specific DMA controller information */
>> 1269 static const struct tegra_dma_chip_data tegra30_dma_chip_data = {
>> 1270         .nr_channels            = 32,
>> 1271         .channel_reg_size       = 0x20,
>> 1272         .max_dma_count          = 1024UL * 64,
>> 1273         .support_channel_pause  = false,
>> 1274         .support_separate_wcount_reg = false,
>> 1275 };
>> 1276
>> 1277 /* Tegra114 specific DMA controller information */
>> 1278 static const struct tegra_dma_chip_data tegra114_dma_chip_data = {
>> 1279         .nr_channels            = 32,
>> 1280         .channel_reg_size       = 0x20,
>> 1281         .max_dma_count          = 1024UL * 64,
>> 1282         .support_channel_pause  = true,
>> 1283         .support_separate_wcount_reg = false,
>> 1284 };
>> 1285
>> 1286 /* Tegra148 specific DMA controller information */
>> 1287 static const struct tegra_dma_chip_data tegra148_dma_chip_data = {
>> 1288         .nr_channels            = 32,
>> 1289         .channel_reg_size       = 0x40,
>> 1290         .max_dma_count          = 1024UL * 64,
>> 1291         .support_channel_pause  = true,
>> 1292         .support_separate_wcount_reg = true,
>> 1293 };
>>
>> You may still say this should be in the DT, but the Tegra maintainers
>> prefer this data in the driver.
> 
> Okay I don't see a a rationale behind this not being in DT, Stephan?

Let me know what you think of Stephen's feedback and if you are OK with
this.

>>>>>> +	dma_cap_set(DMA_SLAVE, tdma->dma_dev.cap_mask);
>>>>>> +	dma_cap_set(DMA_PRIVATE, tdma->dma_dev.cap_mask);
>>>>>> +	dma_cap_set(DMA_CYCLIC, tdma->dma_dev.cap_mask);
>>>>>
>>>>> I think you should not set DMA_SLAVE, do you need caps to be exported. I
>>>>> think that should be exported for cyclic too, let me know if that was the
>>>>> issue?
>>>>>
>>>>
>>>> Why should I not be setting DMA_SLAVE? Should I not be calling
>>>> dma_get_any_slave_channel() in the xlate?
>>>>
>>>> I think that I do want to set DMA_CYCLIC as well to ensure that we check
>>>> that the device->device_prep_dma_cyclic() function pointer is populated
>>>> when registering the DMA controller.
>>>
>>> Only setting DMA_CYCLIC should do, if you see any issues around that please
>>> get back, we cna fix those :)
>>
>> It did not work for me because dma_get_any_slave_channel() wants a DMA
>> device with the DMA_SLAVE bit capability set. So if I remove this above,
>> then requesting the channel fails via dma_get_any_slave_channel() fails.
>> Is there something I don't understand here?
> 
> You should use dma_request_channel() we cleaned up the APIs and recommend to
> use dma_request_channel() for slave usages. See Documentation update in 
> a8135d0d79e9: (dmaengine: core: Introduce new, universal API to request a
> channel)

I had a look at this, but actually, I don't think this is going to work.

Looking at dma_request_channel(), it is going to get a DMA channel that
matches the mask for any DMA controller. In the xlate I already know
which DMA controller I am and I just want one of the channels. The flow
here is ...

dma_request_chan()
  --> of_dma_request_slave_channel()
    --> xlate()
      --> dma_get_any_slave_channel()

There are several other DMA drivers that are calling
dma_get_any_slave_channel() from their xlate function which makes sense
because they are requesting one of their own channels.

I can understand that you wish to consolidate the APIs for requesting a
channel, but it seems to me that you still need to have an API that DMA
controller drivers can call where they can pass their dma_device
structure to ensure you get a channel for the appropriate DMA controller.

Cheers
Jon

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