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Date:   Tue, 31 Mar 2020 19:28:13 +0300
From:   Sergey Semin <Sergey.Semin@baikalelectronics.ru>
To:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>
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 05:25:30PM +0300, Andy Shevchenko wrote:
> 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

Yes. I also intended to do this just by altering the dw_readl() and
dw_writel() methods to work over regmap IO methods if regmap is
available.

> 2/ platform and PCI driver may provide regmap MMIO

Regmap pointer will be also a part of "struct dw_i2c_dev". So if PCI
code intends the regmap-based access to the controller registers, then
it shall just initialize the regmap pointer in the private i2c-designware data
instance of the dw_i2c_dev structure. So, yes, this is also covered by
my solution. Though the PCI code will be left untouched, since I can't
predict a particular regmap-related use-case of it.

> 3/ your glue driver will provide different regmap accessors

I was thinking of developing a more generic version so any platform
with a specific access to the DW I2C register could use it just by supplying
the regmap pointer in the dw_i2c_platform_data structure. Our DW I2C
controller also perfectly fits to the generic i2c-designware-platdrv.c
driver, so implementing an additional glue-layer would be too much seeing
the difference only in the registers mapping.

Let me explain the difference of our solutions. In case of implementing
the glue layer, as you suggest, I would have to do it in a way like the DW PCIe
driver is designed. I would need to move the code of current dw_i2c_plat_probe()
function to a dedicated method named like dw_i2c_plat_init(pdev, !regmap!),
while former method dw_i2c_plat_probe() would just call
dw_i2c_plat_init(pdev, !NULL!). Then I would have to create a dedicated
glue-driver - i2c-designware-bt1drv.c, which would be bound to a
"baikal,t1-sys-i2c" device, try to find a Baikal-T1 System Controller
device node (though this would be just a parent device), then would get
it' syscon regmap handler, then would initialize a dedicated regmap handler to
indirectly access the DW I2C controller register, then it would call the
dw_i2c_plat_init(pdev, !regmap!) method with new regmap handler passed
(though the new regmap passing could be also implemented over the
platform_data pointer). Also seeing you already have a platform-specific
parts in the generic i2c-designware-platdrv.c driver (like ACPI-based
platforms and Microsemi Ocelot SoC), there might raise a necessity to
unpin that specifics to a dedicated method, since my glue-layer
wouldn't need that checks and initializations. Such alteration won't
be that easy to implement and regression errors prone, since I don't have
other platforms to test it.

In case of my solution the whole glue-layer part would be moved to
the MFD-based Baikal-T1 System Controller driver and a generic
platform_data-based interface would be implemented, which would just
need to alter the registers mapping part of the i2c-designware-platdrv.c
driver. Note that that part would need to be fixed in case of any solution.

So comparing these too approaches, I would select a one, which would
need less common code modifications and would provide a generic
solution. As I see it would be a platform_data-based design. What do you
think?

Regards,
-Sergey

> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>