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

From: Wolfram Sang
Date: Thu Feb 14 2019 - 11:53:21 EST

On Tue, Feb 12, 2019 at 11:06:46AM -0800, 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@xxxxxxxxxx>
> Reviewed-by: Dmitry Osipenko <digetx@xxxxxxxxx>
> Tested-by: Dmitry Osipenko <digetx@xxxxxxxxx>
> Signed-off-by: Sowjanya Komatineni <skomatineni@xxxxxxxxxx>

Not directly related to your code, but maybe someone is interested in
fixing this rightful checkpatch comment somewhen later:

WARNING: Avoid crashing the kernel - try using WARN_ON & recovery code rather than BUG() or BUG_ON()
#332: FILE: drivers/i2c/busses/i2c-tegra.c:791:
+ BUG();

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