linux-kernel - RE: [PATCH V9 3/5] i2c: tegra: Add DMA support

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Date:   Sat, 2 Feb 2019 18:32:11 +0000
From:   Sowjanya Komatineni <skomatineni@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Mantravadi Karthik <mkarthik@...dia.com>,
        Shardar Mohammed <smohammed@...dia.com>,
        Timo Alho <talho@...dia.com>
CC:     "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>
Subject: RE: [PATCH V9 3/5] i2c: tegra: Add DMA support

> > This patch adds DMA support for Tegra I2C.
> > 
> > Tegra I2C TX and RX FIFO depth is 8 words. PIO mode is used for 
> > transfer size of the max FIFO depth and DMA mode is used for transfer 
> > size higher than max FIFO depth to save CPU overhead.
> > 
> > PIO mode needs full intervention of CPU to fill or empty FIFO's and 
> > also need to service multiple data requests interrupt for the same 
> > transaction. This adds delay between data bytes of the same transfer 
> > when CPU is fully loaded and some slave devices has internal timeout 
> > for no bus activity and stops transaction to avoid bus hang. DMA mode 
> > is helpful in such cases.
> > 
> > DMA mode is also helpful for Large transfers during downloading or 
> > uploading FW over I2C to some external devices.
> > 
> > Signed-off-by: Sowjanya Komatineni <skomatineni@...dia.com>
> > ---
>
>
> > +static int tegra_i2c_init_dma(struct tegra_i2c_dev *i2c_dev) {
> > +	struct dma_chan *dma_chan;
> > +	u32 *dma_buf;
> > +	dma_addr_t dma_phys;
> > +	int err = 0;
> > +
> > +	if (!IS_ENABLED(CONFIG_TEGRA20_APB_DMA))
> > +		return -ENODEV;
>
> Another detail is that we need to keep older kernels working on T186+ after its device-tree will get the "dmas" property, device-tree changes shall be backwards-compatible with older kernels. Hence we need to check that platform actually wants the APB DMA driver, otherwise T186+ will be failing with -EPROBE_DEFER.

Yes, that will be a separate patch later for adding DMA support for Tegra186 and later chips once we check on GPCDMA upstream