linux-kernel - Re: [PATCH V19 5/7] i2c: tegra: Add DMA support

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Message-ID: <a98faf49-98f5-7b85-8acd-0fd6a4c8b426@nvidia.com>
Date:   Wed, 20 Feb 2019 18:02:05 +0000
From:   Jon Hunter <jonathanh@...dia.com>
To:     Sowjanya Komatineni <skomatineni@...dia.com>,
        <thierry.reding@...il.com>, <mkarthik@...dia.com>,
        <smohammed@...dia.com>, <talho@...dia.com>
CC:     <wsa@...-dreams.de>, <peda@...ntia.se>, <digetx@...il.com>,
        <linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-i2c@...r.kernel.org>
Subject: Re: [PATCH V19 5/7] i2c: tegra: Add DMA support


On 12/02/2019 19:06, Sowjanya Komatineni wrote:
> 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.
> 
> Tegra210 and prior Tegra chips use APBDMA driver which is replaced
> with GPCDMA on Tegra186 and Tegra194.
> This patch uses has_apb_dma flag in hw_feature to differentiate
> DMA driver change between Tegra chipset.
> 
> APBDMA driver is registered from module-init level and this patch
> also has a change to register I2C driver at module-init level
> rather than subsys-init to avoid deferring I2C probe till APBDMA
> driver is registered.
> 
> Acked-by: Thierry Reding <treding@...dia.com>
> Reviewed-by: Dmitry Osipenko <digetx@...il.com>
> Tested-by: Dmitry Osipenko <digetx@...il.com>
> Signed-off-by: Sowjanya Komatineni <skomatineni@...dia.com>

...

> +static int tegra_i2c_init_dma(struct tegra_i2c_dev *i2c_dev)
> +{
> +	struct dma_chan *chan;
> +	u32 *dma_buf;
> +	dma_addr_t dma_phys;
> +	int err;
> +
> +	if (!IS_ENABLED(CONFIG_TEGRA20_APB_DMA) ||
> +	    !i2c_dev->hw->has_apb_dma) {
> +		err = -ENODEV;
> +		goto err_out;
> +	}
> +
> +	chan = dma_request_slave_channel_reason(i2c_dev->dev, "rx");
> +	if (IS_ERR(chan)) {
> +		err = PTR_ERR(chan);
> +		goto err_out;
> +	}
> +
> +	i2c_dev->rx_dma_chan = chan;
> +
> +	chan = dma_request_slave_channel_reason(i2c_dev->dev, "tx");
> +	if (IS_ERR(chan)) {
> +		err = PTR_ERR(chan);
> +		goto err_out;
> +	}
> +
> +	i2c_dev->tx_dma_chan = chan;
> +
> +	dma_buf = dma_alloc_coherent(i2c_dev->dev, i2c_dev->dma_buf_size,
> +				     &dma_phys, GFP_KERNEL | __GFP_NOWARN);
> +	if (!dma_buf) {
> +		dev_err(i2c_dev->dev, "failed to allocate the DMA buffer\n");
> +		err = -ENOMEM;
> +		goto err_out;
> +	}
> +
> +	i2c_dev->dma_buf = dma_buf;
> +	i2c_dev->dma_phys = dma_phys;
> +	return 0;
> +
> +err_out:
> +	tegra_i2c_release_dma(i2c_dev);
> +	if (err != -EPROBE_DEFER) {
> +		dev_err(i2c_dev->dev, "cannot use DMA: %d\n", err);
> +		dev_err(i2c_dev->dev, "fallbacking to PIO\n");
> +		return 0;
> +	}
I think that the above should be a dev_dbg print or re-worked in someway
because now for Tegra194 which does not have an APB DMA I see ...

[       6.093234] ERR KERN tegra-i2c 31c0000.i2c: cannot use DMA: -19
[       6.096847] ERR KERN tegra-i2c 31c0000.i2c: falling back to PIO

Given that the APB DMA is not supported for Tegra186/Tegra194, there is
no point in printing these error messages. Now it looks like something
is wrong but really it is not :-(

Cheers
Jon

-- 
nvpublic