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

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Date:   Sun, 3 Feb 2019 19:48:09 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Sowjanya Komatineni <skomatineni@...dia.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

02.02.2019 21:32, Sowjanya Komatineni пишет:
>>> 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
> 
> 

Sure, and there is a requirement in upstream to keep older kernel versions working with the later device-tree updates, as I pointed above. Please include the ".has_apb_dma checking" that I'm suggesting into the next version.