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    SubjectRE: [PATCH RESEND 1/2] i2c: tegra: Add GPCDMA support
    Date
    > 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().
    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.

    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.

    Regards,
    Akhil
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