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

[Akhil R <akhilrajeev@nvidia.com>]

> 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).
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().

Regards,
Akhil