linux-kernel - RE: [PATCH V2 3/4] i2c: tegra: Add DMA Support

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Message-ID: <BYAPR12MB33987E81045953368D0113A2C29B0@BYAPR12MB3398.namprd12.prod.outlook.com>
Date:   Fri, 25 Jan 2019 22:22:40 +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 V2 3/4] i2c: tegra: Add DMA Support



> >>>>> +	if (i2c_dev->has_dma) {
> >>>>> +		ret = tegra_i2c_init_dma_param(i2c_dev, true);
> >>>>> +		if (ret == -EPROBE_DEFER)
> >>>>> +			goto disable_div_clk;
> >>>>> +		ret = tegra_i2c_init_dma_param(i2c_dev, false);
> >>>>> +		if (ret == -EPROBE_DEFER)
> >>>>> +			goto disable_div_clk;
> >>>>
> >>>> So tegra_i2c_init_dma_param() could fail, printing a error message, and probe will succeed? If allocation fails during the driver's probe, then just fail the probe. Please give the rationale.
> >>>
> >>> If APB DMA probe doesn’t happen prior to tegra i2c, DMA is not available by the time tegra_init_dma_param tries to request slave channel and in those cases dma_request_slave_channel_reason will return EPROBE_DEFER for tegra I2C probe to retry
> >>>
> >>> In case if DMA is available but DMA buffer allocation fails, then tegra_i2c_init_dma_param returns ENOMEM and probe also fails returning same ENOMEM
> >>
> >> Is that what you're going to change in the next version? Your current variant of the code doesn't fail the probe on ENOMEM and there is duplicated attempt to invoke tegra_i2c_init_dma_param() during the transfer.
> > 
> > Sorry correction to my previous reply.  If DMA buffer allocation fails, tegra_i2c_init_dma_param returns ENOMEM but probe will succeed as i2c transaction need to happen during  boot for some platform device programming for successful boot and they use PIO > mode as xfer bytes is less and deferring i2c probe for ENOMEM causes boot to fail so during probe EPROBE_DEFER is only taken care.
> > 
> > Re-attempt of tegra_i2c_init_dma_param in xfer happens only if no successful DMA channel allocation happens prior to that ( during probe in case of ENOMEM).
> > DMA mode is mainly for large transfer, and i2c xfer returning failure due to failing DMA buffer allocation causes boot to hang as platform device programming need to happen which doesn’t need to use DMA.
> > Will fix this and will send updated patch to reattempt DMA request and buffer allocation during DMA mode transfer and will return fail for DMA mode I2C transfer...
>
> 1) The chance to get ENOMEM during is miserable and likely that there are much bigger problems in this case. Hence just fail the probe if tegra_i2c_init_dma_param() fails with any error.

Agree. 

> 2) What is that super-critical platform device? It is possible to compile tegra-i2c as a kernel module and seems nothing enforces CONFIG_I2C_TEGRA=y.
Some of the i2c device  access happens during boot for some tegra platforms like jetson tx1, tx2.. (Eg: I2C GPIO expanders, PMIC access...)

> 3) After applying these patches I2C transfers are failing on Tegra20 with "tegra-i2c 7000c400.i2c: Failed to allocate the DMA buffer". 
This patch should not cause DMA buffer allocation failure to happen. But if allocation fails (it couldn’t allocate contiguous physical memory for some reason), then transfers will definitely fail with this patch.

Will send updated patch with not re-attempt and in probe to return on error so if it will either defer or fail in case of ENOMEM.