linux-kernel - RE: [PATCH RESEND 1/2] i2c: tegra: Add GPCDMA support

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Date:   Tue, 23 Aug 2022 05:17:17 +0000
From:   Akhil R <akhilrajeev@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>,
        Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "christian.koenig@....com" <christian.koenig@....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>,
        "sumit.semwal@...aro.org" <sumit.semwal@...aro.org>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "wsa@...nel.org" <wsa@...nel.org>
Subject: RE: [PATCH RESEND 1/2] i2c: tegra: Add GPCDMA support

> 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