Message-ID: <0b870d47-8e5e-1a5b-3ef6-9310faa63093@gmail.com>
Date:   Thu, 31 Jan 2019 19:31:17 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Sowjanya Komatineni <skomatineni@...dia.com>, jonathanh@...dia.com,
        mkarthik@...dia.com, smohammed@...dia.com, talho@...dia.com,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-i2c@...r.kernel.org
Subject: Re: [PATCH V7 3/5] i2c: tegra: Add DMA Support

31.01.2019 19:27, Dmitry Osipenko пишет:
> 31.01.2019 19:01, Thierry Reding пишет:
>> On Thu, Jan 31, 2019 at 06:02:45PM +0300, Dmitry Osipenko wrote:
>>> 31.01.2019 17:43, Thierry Reding пишет:
>>>> On Thu, Jan 31, 2019 at 05:06:18PM +0300, Dmitry Osipenko wrote:
>>>>> 31.01.2019 15:06, Thierry Reding пишет:
>>>>>> On Thu, Jan 31, 2019 at 03:05:48AM +0300, Dmitry Osipenko wrote:
>>>>>>> 30.01.2019 19:01, Sowjanya Komatineni пишет:
>>>>>> [...]
>>>>>>>> diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c
>>>>>> [...]
>>>>>>>> +		return -EIO;
>>>>>>>> +	}
>>>>>>>> +
>>>>>>>> +	dma_desc->callback = tegra_i2c_dma_complete;
>>>>>>>> +	dma_desc->callback_param = i2c_dev;
>>>>>>>> +	dmaengine_submit(dma_desc);
>>>>>>>> +	dma_async_issue_pending(chan);
>>>>>>>> +	return 0;
>>>>>>>> +}
>>>>>>>> +
>>>>>>>> +static int tegra_i2c_init_dma_param(struct tegra_i2c_dev *i2c_dev,
>>>>>>>> +				    bool dma_to_memory)
>>>>>>>> +{
>>>>>>>> +	struct dma_chan *dma_chan;
>>>>>>>> +	u32 *dma_buf;
>>>>>>>> +	dma_addr_t dma_phys;
>>>>>>>> +	int ret;
>>>>>>>> +	const char *chan_name = dma_to_memory ? "rx" : "tx";
>>>>>>>
>>>>>>> What about to move out chan_name into the function arguments?
>>>>>>
>>>>>> That opens up the possibility of passing dma_to_memory = true and
>>>>>> chan_name as "tx" and create an inconsistency.
>>>>>>
>>>>>>>> @@ -884,6 +1187,8 @@ static void tegra_i2c_parse_dt(struct tegra_i2c_dev *i2c_dev)
>>>>>>>>  
>>>>>>>>  	i2c_dev->is_multimaster_mode = of_property_read_bool(np,
>>>>>>>>  			"multi-master");
>>>>>>>> +
>>>>>>>> +	i2c_dev->has_dma = of_property_read_bool(np, "dmas");
>>>>>>>
>>>>>>> Not only the existence of "dmas" property defines whether DMA is available. DMA subsystem could be disabled in the kernels configuration.
>>>>>>>
>>>>>>> Hence there is a need to check for DMA driver presence in the code:
>>>>>>>
>>>>>>> 	if (IS_ENABLED(CONFIG_TEGRA20_APB_DMA))
>>>>>>> 		i2c_dev->has_dma = of_property_read_bool(np, "dmas");
>>>>>>
>>>>>> Do we even need the ->has_dma at all? We can just go ahead and request
>>>>>> the channels at probe time and respond accordingly. If there's no dmas
>>>>>> property in DT, dma_request_slave_channel_reason() returns an error so
>>>>>> we can just deal with it at that time.
>>>>>>
>>>>>> So if we get -EPROBE_DEFER we can propagate that, for any other errors
>>>>>> we can simply fallback to PIO. Or perhaps we want to restrict fallback
>>>>>> to PIO for -ENODEV?
>>>>>>
>>>>>> I wouldn't want to add an IS_ENABLED(CONFIG_TEGRA20_APB_DMA) in here.
>>>>>> The purpose of these subsystems it to abstract all of that away.
>>>>>> Otherwise we could just as well use custom APIs, if we're tying together
>>>>>> drivers in this way anyway.
>>>>>
>>>>> DMA API doesn't fully abstract the dependencies between drivers, hence
>>>>> I disagree.
>>>>
>>>> Why not? The dependency we're talking about here is a runtime dependency
>>>> rather than a build time dependency. Kconfig is really all about build-
>>>> time dependencies.
>>>
>>> My understanding is that Kconfig is also about runtime dependencies,
>>> do you know if it's explicitly documented anywhere?
>>
>> I don't think it's explicitly documented, just a common practice that
>> I've seen applied multiple times over the years. A quick grep through
>> the drivers/ subdirectory confirms that it's not typical to have this
>> sort of dependency in the code.
>>
>> Similarly, Kconfig uses select primarily to pull in dependencies that
>> are in the form of helper libraries and such. Occasionally you'll have
>> some ARCH_* option select a couple of features, or even drivers, but
>> that is mostly a shortcut to explicitly having to list the essentials
>> in a defconfig.
>>
>> Another reason why it's not good to model these runtime dependencies in
>> Kconfig is because they unnecessarily restrict the driver. For example,
>> if you want to build a specialized Linux binary for Tegra186, you will
>> certainly want to include the i2c-tegra driver. At the same time you
>> won't want to include the APB DMA driver because it doesn't exist on
>> Tegra186. Instead you'd want the (non-existent) GPC DMA driver. select
>> on the APB DMA driver will unconditionally pull in the driver, depends
>> will only allow you to build i2c-tegra if the APB DMA driver is also
>> enabled and the conditional in the code may lead to not using DMA
>> because the APB DMA driver is not available. So you'd have to modify the
>> i2c-tegra driver to take into account the GPC DMA driver.
>>
>>>>>>> Also Tegra I2C driver should select DMA driver in Kconfig to make DMA
>>>>>>> driver built-in when I2C driver is built-in:
>>>>>>
>>>>>> I don't think there's a requirement for that. The only dependency we
>>>>>> really have here is the one on the DMA engine API. Since dmaengine.h
>>>>>> already provides dummy implementations, there's really no need for
>>>>>> us to have the dependency. If the DMA engine API is completely disabled,
>>>>>> a call to dma_request_slave_channel_reason() will return -ENODEV and we
>>>>>> should just deal with that the same way we would if there was no "dmas"
>>>>>> property present.
>>>>>
>>>>> In my opinion it is much better to avoid I2C driver probe failing with
>>>>> -EPROBE_DEFER if we could. It's just one line in code and one in
>>>>> Kconfig.. really. 
>>>>
>>>> The problem is that from a theoretical point of view we don't know that
>>>> APB DMA is the provider for the DMA channels. This provider could be a
>>>> completely different device on a different Tegra generation (in fact,
>>>> the DMA engine on Tegra186 is a different one, so we'd have to add that
>>>> to the list of checks to make sure we don't disable DMA there). And the
>>>> fact that we're tightly integrated is really only by accident. We could
>>>> have the same situation on a SoC that incorporates IP from multiple
>>>> different sources and multiple combinations thereof as well, so how
>>>> would you want to deal with those cases?
>>>>
>>>> Agreed, failing with -EPROBE_DEFER is suboptimal in that case, but that
>>>> is really more of an integration problem. Ideally of course there'd be
>>>> some way for the DMA engine subsystem to know that the provider for the
>>>> given device node will never show up and give us -ENODEV instead, but,
>>>> alas, I don't even think that would be possible. That's the price to pay
>>>> for abstraction.
>>>
>>> It's not a big problem to solve for this case, there is
>>> of_machine_is_compatible(). To me it's more a question about the will
>>> to invest some extra effort to support all of possible combinations.
>>> If there is no such will, then at least those unpopular combinations
>>> shouldn't hurt and thus it should make sense to add an explicit
>>> build-dependency on the DMA drivers.
>>
>> I think we're arguing about the same thing, only coming at it from
>> different angles. For me "all possible combinations" also includes the
>> case where you want to be able to run the driver with DMA if the APB DMA
>> is not enabled. And I similarly want to be able to run without DMA if
>> the APB DMA is enabled (by explicitly removing dmas from DT for
>> example). It just seems that we can't have it both ways.
>>
>> Also the i2c-tegra driver can perfectly well function without DMA
>> support (it's done so ever since it was first merged). Keeping existing
>> functionality shouldn't require the addition of another driver.
>>
>> Given the deadlock, I think I'd prefer the option of adding the
>> conditional in the code. I think that's the most accurate description of
>> the dependency, even though ideally it would be handled transparently by
>> the DMA engine API. Would that be an acceptable compromise?
> 
> Adding conditional to the code is not enough. Tegra I2C driver could be built-in, while APB DMA driver is a loadable module, hence Tegra I2C will fail to probe with -EPROBE_DEFER. Tegra I2C must select all of the relevant DMA drivers to avoid that situation. Later on it shouldn't be a problem to add .has_gpc_dma to the tegra_i2c_hw_feature and then check in the code whether corresponding DMA driver is enabled or not in the kernel's config.
> 
> Combining the code checking with the Kconfig selection that I'm suggesting covers all of possible combinations, otherwise please give me an explicit example when it could fail.
> 

Actually .has_gpc_dma need to be added and handled right now to maintain backwards compatibility with the future DT updates.

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