Message-ID: <c9ba2629-fc81-cefd-0d6d-991084781ec3@collabora.com>
Date:   Tue, 23 Aug 2022 11:45:00 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Akhil R <akhilrajeev@...dia.com>,
        Dmitry Osipenko <digetx@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "wsa@...nel.org" <wsa@...nel.org>
Cc:     "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "christian.koenig@....com" <christian.koenig@....com>,
        "sumit.semwal@...aro.org" <sumit.semwal@...aro.org>
Subject: Re: [PATCH RESEND 1/2] i2c: tegra: Add GPCDMA support

On 8/23/22 11:39, Dmitry Osipenko wrote:
> 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
> 

Although, probably MODULE_SOFTDEP won't work for a built-in driver. In
that case, change Tegra I2C kconfig to depend on the DMA driver.

-- 
Best regards,
Dmitry

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