On 8/23/22 11:39, Dmitry Osipenko wrote: > On 8/23/22 08:17, Akhil R wrote: >>> 22.08.2022 23:33, Dmitry Osipenko пишет: >>>> 22.08.2022 13:29, Akhil R пишет: >>>>>> On 8/22/22 09:56, Akhil R wrote: >>>>>>>> 19.08.2022 18:15, Dmitry Osipenko пишет: >>>>>>>>> 19.08.2022 15:23, Akhil R пишет: >>>>>>>>>> if (of_device_is_compatible(np, "nvidia,tegra210-i2c-vi")) >>>>>>>>>> i2c_dev->is_vi = true; >>>>>>>>>> + else >>>>>>>>>> + i2c_dev->dma_support = !!(of_find_property(np, "dmas", >>>>>>>>>> + NULL)); >>>>>>>>> >>>>>>>>> 1. You leak the np returned by of_find_property(). >>>>>>>>> >>>>>>>>> 2. There is device_property_read_bool() for this kind of >>>>>>>>> property-exists checks. >>>>>>> Okay. I went by the implementation in of_dma_request_slave_channel() to >>>>>>> check 'dmas'. >>>>>>> >>>>>>>>> >>>>>>>>> 3. If "dmas" is missing in DT, then dma_request_chan() should return >>>>>>>>> NULL and everything will work fine. I suppose you haven't tried to >>>>>>>>> test this code. >>>>>>>> >>>>>>>> Although, no. It should return ERR_PTR(-ENODEV) and then you should >>> check >>>>>>>> the return code. >>>>>>> Yes. Agree that it is more agnostic to check for ERR_PTR(-ENODEV). But >>> since I >>>>>>> call tegra_init_dma() for every large transfer until DMA is initialized, >>> wouldn't >>>>>>> it be better to have a flag inside the driver so that we do not have to go >>>>>> through >>>>>>> so many functions for every attempted DMA transaction to find out that >>> the >>>>>> DT >>>>>>> properties don't exist? >>>>>>> >>>>>>> Shall I just put i2c_dev->dma_support = true here since DMA is supported >>> by >>>>>>> hardware? It would turn false if dma_request_chan() returns something >>> other >>>>>>> than -EPROBE_DEFER. >>>>>>> >>>>>>> if (of_device_is_compatible(np, "nvidia,tegra210-i2c-vi")) >>>>>>> i2c_dev->is_vi = true; >>>>>>> + else >>>>>>> + i2c_dev->dma_support = true; >>>>>> >>>>>> The code already has dma_mode for that. I don't see why another variable >>>>>> is needed. >>>>>> >>>>>> Either add new generic dma_request_chan_optional() that will return NULL >>>>>> if channel is not available and make Tegra I2C driver to use it, or >>>>>> handle the error code returned by dma_request_chan(). >>>>> >>>>> Let me elaborate my thoughts. >>>>> >>>>> The function tegra_i2c_init_dma() is also called inside tegra_i2c_xfer_msg() if >>>>> DMA is not initialized before, i.e. if (!i2c_dev->dma_buf). >>>> >>>> This is not true >>>> >>>> i2c_dev->dma_mode=false if !i2c_dev->dma_buf and that's it >>>> >>>> https://elixir.bootlin.com/linux/v6.0-rc2/source/drivers/i2c/busses/i2c- >>> tegra.c#L1253 >>>> >>>> tegra_i2c_init_dma() is invoked only during probe >>>> >>>>> So, if suppose there is no DT entry for dmas, the driver would have to go take >>> the >>>>> path tegra_i2c_init_dma() -> dma_request_chan() -> of_*() apis -> ... and >>> then figure >>>>> out that DMA is not supported. This would happen for each transfer of size >>> larger than >>>>> I2C_PIO_MODE_PREFERRED_LEN. >>>>> >>>>> To avoid this, I am looking for a variable/flag which can indicate if the driver >>> should attempt >>>>> to configure DMA or not. I didn't quite get the idea if dma_mode can be >>> extended to support >>>>> this, because it is updated based on xfer_size on each transfer. My idea of >>> i2c_dev->dma_support >>>>> is that it will be constant after the probe(). >>> >>> I see now that it's you added tegra_i2c_init_dma() to >>> tegra_i2c_xfer_msg(). And tegra_i2c_init_dma() already falls back to PIO >>> if DMA is unavailable. >>> >>> I don't remember why !IS_ENABLED(CONFIG_TEGRA20_APB_DMA) was added >>> to >>> tegra_i2c_init_dma(), but if dma_request_chan() returns -EPROBE_DEFER >>> when there is no DMA channel available at all, then you should fix it. >>> >>> Trying to initialize DMA during transfer if it failed to initialize >>> during probe is a wrong approach. DMA must be initialized only once >>> during probe. Please make the probe to work properly. >> >> What I am trying for is to have a mechanism that doesn't halt the i2c transfers >> till DMA is available. Also, I do not want to drop DMA because it was unavailable >> during probe(). > > Why is it unavailable? Sounds like you're not packaging kernel properly. > >> This situation is sure to hit if we have I2C driver as built in and DMA driver as a >> module. In such cases, I2C will never be able to use the DMA. > > For Tegra I2C built-in + DMA driver module you should add the dma.ko to > initramfs and then it will work. This is a common practice for many > kernel drivers. > > It's also similar to a problem with firmware files that must be > available to drivers during boot, > >> Another option I thought about was to request and free DMA channel for each >> transfer, which many serial drivers already do. But I am a bit anxious if that will >> increase the latency of transfer. > > Perhaps all you need to do is to add MODULE_SOFTDEP to Tegra I2C driver > like we did it for the EMC driver [1]. > > [1] > https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=14b43c20c283de36131da0cb44f3170b9ffa7630 > Although, probably MODULE_SOFTDEP won't work for a built-in driver. In that case, change Tegra I2C kconfig to depend on the DMA driver. -- Best regards, Dmitry
