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Date:   Tue, 31 Mar 2020 17:25:30 +0300
From:   Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To:     Sergey Semin <Sergey.Semin@baikalelectronics.ru>
Cc:     Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>,
        Maxim Kaurkin <Maxim.Kaurkin@baikalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@baikalelectronics.ru>,
        Ramil Zaripov <Ramil.Zaripov@baikalelectronics.ru>,
        Ekaterina Skachko <Ekaterina.Skachko@baikalelectronics.ru>,
        Vadim Vlasov <V.Vlasov@baikalelectronics.ru>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Paul Burton <paulburton@kernel.org>,
        Ralf Baechle <ralf@linux-mips.org>,
        Jarkko Nikula <jarkko.nikula@linux.intel.com>,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Wolfram Sang <wsa@the-dreams.de>, linux-i2c@vger.kernel.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/6] i2c: designeware: Add Baikal-T1 SoC DW I2C specifics
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On Tue, Mar 31, 2020 at 02:48:24PM +0300, Sergey Semin wrote:
> Hello Andy,
> 
> Finally I've thought this through reasonably conformed with the changes
> requested in the framework of the other patchsets. My comments are
> below.
> 
> On Fri, Mar 06, 2020 at 03:54:45PM +0200, Andy Shevchenko wrote:
> > First of all, I got only 3 out of 6 patches. Are you sure you properly prepared
> > the series?
> > 
> > On Fri, Mar 06, 2020 at 04:19:49PM +0300, Sergey.Semin@baikalelectronics.ru wrote:
> > > From: Serge Semin <fancer.lancer@gmail.com>
> > 
> > Same comment as per DMA series, try next time to link the cover letter to the
> > series correctly.
> > 
> > > There are three DW I2C controllers embedded into the Baikal-T1 SoC. Two
> > > of them are normal with standard DW I2C IP-core configurations and registers
> > > accessible over normal MMIO space - so they are acceptable by the available
> > > DW I2C driver with no modification.
> > 
> > > But there is a third, which is a bit
> > > different. Its registers are indirectly accessed be means of "command/data
> > > in/data out" registers tuple. In order to have it also supported by the DW
> > > I2C driver, we must modify the code a bit. This is a main purpose of this
> > > patchset.
> > > 
> > > First of all traditionally we replaced the legacy plain text-based dt-binding
> > > file with yaml-based one. Then we found and fixed a bug in the DW I2C FIFO size
> > > detection algorithm which tried to do it too early before dw_readl/dw_writel
> > > methods could be used.
> > 
> > So far so good (looks like, I think colleagues of mine and myself will review
> > individual patches later on).
> > 
> > > Finally we introduced a platform-specific flag
> > > ACCESS_INDIRECT, which would enable the indirect access to the DW I2C registers
> > > implemented for one of the Baikal-T1 SoC DW I2C controllers. See the commit
> > > message of the corresponding patch for details.
> > 
> > This is quite questionable. In Intel SoCs we have indirect I?C controllers to
> > access (inside PMIC, for example). The approach used to do that is usually to
> > have an IPC mechanism and specific bus controller driver. See i2c-cht-wc.c for
> > instance.
> > 
> > I'm not sure if it makes a lot of duplication and if actually switching I?C
> > DesignWare driver to regmap API will solve it. At least that is the second
> > approach I would consider.
> > 
> > But I'll wait others to comment on this. We have to settle the solution before
> > going further.
> > 
> 
> As I see the others have not comments.) Anyway I see your point and having the
> regmap-based interface might be better than the approach I've suggested
> in this patchset particularly seeing that our DW i2c IP registers are
> hidden behind a system controller register space.
> 
> In order to follow your proposition to create a dedicated regmap and to supply
> it to the DW i2c driver, I have to redevelop not only this patchset, but
> also an adjacent drivers. In particular the changes will concern the
> MFD-based System Controller driver (which will instantiate this DW i2c
> controller device), Clocks Control Unit drivers set, and a few
> others. The whole alteration I described in the RFC:
> https://lkml.org/lkml/2020/3/22/393
> You've been in Cc there, so fill free to send your comments regarding
> the changes I suggested. Though this time I hope the solution will
> satisfy everyone, who had issues with patchsets I've recently sent.
> 
> Getting back to your comment in the framework of this patchset. The approach
> used for CHT Whiskey Cove i2c isn't fully suitable in our case for
> the reason of the DW I2C controller nature. DW I2C controller is a generic
> controller and used on many different platforms, while AFAICS CHT Whiskey Cove
> I2C is the SoC-specific used to access a charger-IC. So in the former case we
> may have an arbitrary set of i2c-slaves connected to the controller on
> different platforms, while on the latter one - there is a fixed set of
> slaves. In addition due to the same reason the DW I2C IP might be
> embedded into different sub-blocks on different platforms, while the CHT
> Whiskey Cove I2C is known to be a part of Intel CHT WC SoC PMIC.
> For instance Baikal-T1 SoC has one DW I2C controller embedded into the
> System Controller with indirectly accessible registers and two DW I2C
> interfaces with normal memory mapped registers. Due to this in case of DW I2C
> driver we can't just "suck" the regmap out from a parental MFD or
> anywhere else as it's done in the CHT Whiskey Cove I2C driver, but instead
> we should somehow supply a regmap pointer to the driver.
> 
> Taking into account all of these we can utilize a combined approach
> implemented in ./drivers/i2c/busses/i2c-cht-wc.c and
> drivers/mfd/intel_quark_i2c_gpio.c . I'll add a regmap pointer field to the
> "struct dw_i2c_platform_data" structure, so in case if there is no
> IORESOURCE_MEM resources available (platform_get_resource() fails), we
> try to get a regmap pointer from the platform data. If there is no valid
> regmap available, then completely fail the driver probe procedure. Though
> due to this alteration I'll have to change the
> dw_i2c_platform_data.i2c_scl_freq field usage a bit. In case if it's
> zero, then call i2c_parse_fw_timings(). This won't hurt ACPI or dt-less
> platforms, but will let us cover a case when regmap is set while i2c
> clock frequency is supposed to be taken from the kernel firmware (like
> dtb, etc).
> 
> So if you are Ok with this, I'll send a v2 patchset with corresponding
> alteration implemented.

I was thinking about something like this:

1/ core driver (library + master + slave) is converted to use regmap
2/ platform and PCI driver may provide regmap MMIO
3/ your glue driver will provide different regmap accessors

-- 
With Best Regards,
Andy Shevchenko