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Date:   Tue, 23 Aug 2022 11:39:56 +0300
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Subject: Re: [PATCH RESEND 1/2] i2c: tegra: Add GPCDMA support
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To:     Akhil R <akhilrajeev@nvidia.com>,
        Dmitry Osipenko <digetx@gmail.com>,
        "robh+dt@kernel.org" <robh+dt@kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "christian.koenig@amd.com" <christian.koenig@amd.com>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        Laxman Dewangan <ldewangan@nvidia.com>,
        "linux-i2c@vger.kernel.org" <linux-i2c@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-tegra@vger.kernel.org" <linux-tegra@vger.kernel.org>,
        "sumit.semwal@linaro.org" <sumit.semwal@linaro.org>,
        "thierry.reding@gmail.com" <thierry.reding@gmail.com>,
        "wsa@kernel.org" <wsa@kernel.org>
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From:   Dmitry Osipenko <dmitry.osipenko@collabora.com>
In-Reply-To: <SJ1PR12MB63397BBD4EF314A742680F2CC0709@SJ1PR12MB6339.namprd12.prod.outlook.com>
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On 8/23/22 08:17, Akhil R wrote:
>> 22.08.2022 23:33, Dmitry Osipenko пишет:
>>> 22.08.2022 13:29, Akhil R пишет:
>>>>> On 8/22/22 09:56, Akhil R wrote:
>>>>>>> 19.08.2022 18:15, Dmitry Osipenko пишет:
>>>>>>>> 19.08.2022 15:23, Akhil R пишет:
>>>>>>>>>      if (of_device_is_compatible(np, "nvidia,tegra210-i2c-vi"))
>>>>>>>>>              i2c_dev->is_vi = true;
>>>>>>>>> +    else
>>>>>>>>> +            i2c_dev->dma_support = !!(of_find_property(np, "dmas",
>>>>>>>>> + NULL));
>>>>>>>>
>>>>>>>> 1. You leak the np returned by of_find_property().
>>>>>>>>
>>>>>>>> 2. There is device_property_read_bool() for this kind of
>>>>>>>> property-exists checks.
>>>>>> Okay. I went by the implementation in of_dma_request_slave_channel() to
>>>>>> check 'dmas'.
>>>>>>
>>>>>>>>
>>>>>>>> 3. If "dmas" is missing in DT, then dma_request_chan() should return
>>>>>>>> NULL and everything will work fine. I suppose you haven't tried to
>>>>>>>> test this code.
>>>>>>>
>>>>>>> Although, no. It should return ERR_PTR(-ENODEV) and then you should
>> check
>>>>>>> the return code.
>>>>>> Yes. Agree that it is more agnostic to check for ERR_PTR(-ENODEV). But
>> since I
>>>>>> call tegra_init_dma() for every large transfer until DMA is initialized,
>> wouldn't
>>>>>> it be better to have a flag inside the driver so that we do not have to go
>>>>> through
>>>>>> so many functions for every attempted DMA transaction to find out that
>> the
>>>>> DT
>>>>>> properties don't exist?
>>>>>>
>>>>>> Shall I just put i2c_dev->dma_support = true here since DMA is supported
>> by
>>>>>> hardware? It would turn false if dma_request_chan() returns something
>> other
>>>>>> than -EPROBE_DEFER.
>>>>>>
>>>>>>       if (of_device_is_compatible(np, "nvidia,tegra210-i2c-vi"))
>>>>>>               i2c_dev->is_vi = true;
>>>>>>  +    else
>>>>>>  +            i2c_dev->dma_support = true;
>>>>>
>>>>> The code already has dma_mode for that. I don't see why another variable
>>>>> is needed.
>>>>>
>>>>> Either add new generic dma_request_chan_optional() that will return NULL
>>>>> if channel is not available and make Tegra I2C driver to use it, or
>>>>> handle the error code returned by dma_request_chan().
>>>>
>>>> Let me elaborate my thoughts.
>>>>
>>>> The function tegra_i2c_init_dma() is also called inside tegra_i2c_xfer_msg() if
>>>> DMA is not initialized before, i.e. if (!i2c_dev->dma_buf).
>>>
>>> This is not true
>>>
>>> i2c_dev->dma_mode=false if !i2c_dev->dma_buf and that's it
>>>
>>> https://elixir.bootlin.com/linux/v6.0-rc2/source/drivers/i2c/busses/i2c-
>> tegra.c#L1253
>>>
>>> tegra_i2c_init_dma() is invoked only during probe
>>>
>>>> So, if suppose there is no DT entry for dmas, the driver would have to go take
>> the
>>>> path tegra_i2c_init_dma() -> dma_request_chan() -> of_*() apis -> ... and
>> then figure
>>>> out that DMA is not supported. This would happen for each transfer of size
>> larger than
>>>> I2C_PIO_MODE_PREFERRED_LEN.
>>>>
>>>> To avoid this, I am looking for a variable/flag which can indicate if the driver
>> should attempt
>>>> to configure DMA or not. I didn't quite get the idea if dma_mode can be
>> extended to support
>>>> this, because it is updated based on xfer_size on each transfer. My idea of
>> i2c_dev->dma_support
>>>> is that it will be constant after the probe().
>>
>> I see now that it's you added tegra_i2c_init_dma() to
>> tegra_i2c_xfer_msg(). And tegra_i2c_init_dma() already falls back to PIO
>> if DMA is unavailable.
>>
>> I don't remember why !IS_ENABLED(CONFIG_TEGRA20_APB_DMA) was added
>> to
>> tegra_i2c_init_dma(), but if dma_request_chan() returns -EPROBE_DEFER
>> when there is no DMA channel available at all, then you should fix it.
>>
>> Trying to initialize DMA during transfer if it failed to initialize
>> during probe is a wrong approach. DMA must be initialized only once
>> during probe. Please make the probe to work properly.
> 
> What I am trying for is to have a mechanism that doesn't halt the i2c transfers
> till DMA is available. Also, I do not want to drop DMA because it was unavailable
> during probe().

Why is it unavailable? Sounds like you're not packaging kernel properly.

> This situation is sure to hit if we have I2C driver as built in and DMA driver as a 
> module. In such cases, I2C will never be able to use the DMA.

For Tegra I2C built-in + DMA driver module you should add the dma.ko to
initramfs and then it will work. This is a common practice for many
kernel drivers.

It's also similar to a problem with firmware files that must be
available to drivers during boot,

> Another option I thought about was to request and free DMA channel for each
> transfer, which many serial drivers already do. But I am a bit anxious if that will
> increase the latency of transfer.

Perhaps all you need to do is to add MODULE_SOFTDEP to Tegra I2C driver
like we did it for the EMC driver [1].

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=14b43c20c283de36131da0cb44f3170b9ffa7630

-- 
Best regards,
Dmitry