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 -- Best regards, Dmitry
