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Date:   Mon, 22 Aug 2022 23:33:23 +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 <dmitry.osipenko@collabora.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>
References: <20220819122313.40445-1-akhilrajeev@nvidia.com>
 <20220819122313.40445-2-akhilrajeev@nvidia.com>
 <20281ca7-e597-7030-4861-5f9a3594726d@gmail.com>
 <89a746fd-a98e-3147-7811-33c5051c2b6d@gmail.com>
 <SJ1PR12MB6339FC1F82EB1BB7417E533BC0719@SJ1PR12MB6339.namprd12.prod.outlook.com>
 <ebb0764f-db92-d69d-49ac-151f4e3e0b8a@collabora.com>
 <SJ1PR12MB63396DC508F63807F1CE9901C0719@SJ1PR12MB6339.namprd12.prod.outlook.com>
From:   Dmitry Osipenko <digetx@gmail.com>
In-Reply-To: <SJ1PR12MB63396DC508F63807F1CE9901C0719@SJ1PR12MB6339.namprd12.prod.outlook.com>
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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().