[RFC 0/2] Add support for Meson MX "SDIO" MMC driver

[ulf.hansson@linaro.org (Ulf Hansson)]

On 10 May 2017 at 21:22, Martin Blumenstingl
<martin.blumenstingl@googlemail.com> wrote:
> Hi Ulf,
>
> On Wed, May 10, 2017 at 10:44 AM, Ulf Hansson <ulf.hansson@linaro.org> wrote:
>> On 6 May 2017 at 19:18, Martin Blumenstingl
>> <martin.blumenstingl@googlemail.com> wrote:
>>> This is the successor to Carlo Caione's "Add support for Amlogic Meson
>>> MMC driver" series (v5) from [0].
>>>
>>> I would like you to specifically review:
>>> - whether I've (ab)used the MMC framework properly (as this is my first
>>>   "larger" contribution to an MMC driver)
>>
>> I take a look soonish.
> thank you!
>
>>> - I think I have improved the locking compared to Carlo's version,
>>>   however I'd still like feedback on whether this looks sane now or if I
>>>   can improve that even further
>>>
>>> (notable) changes since Carlo's latest version are:
>>> - renamed the driver to meson-mx-sdio (Amlogic's reference kernel calls
>>>   the driver "aml_sdio" as there is a second MMC controller in these SoCs
>>>   which they call the "SDHC controller"). do the same with our driver to
>>
>> I don't like to renaming drivers, just because there are a reference
>> kernel using a different name.
>>
>> What's is really the difference between controllers? Why do they have
>> two variants?
> the driver from this thread is for the "SDSC/SDHC/SDXC card and SDIO
> interface with 1-bit and 4-bit data bus width supporting spec version
> 2.x/3.x/4.x DS/HS modes up to UHS-I SDR50" (quote taken from the S805
> datasheet: [0])
> the "other" controller is an "eMMC and MMC card interface with
> 1/4/8-bit data bus width supporting spec version 4.4x/4.5x HS200 (up
> to 100MHz clock), compatible with standard iNAND interface" (again,
> quote taken from the S805 datasheet: [0])
>
>>
>> Can they be managed by the same driver?
>>
>>>   avoid confusion once we add support for the second controller (which uses
>>>   a completely different register layout)
>>
>> Besides that, do they behave differently in some other way?
> both drivers/controllers have a totally different register layout - I
> don't see any way how they both could be handled by one driver (the
> registers for the controllers from this series are not part of the
> documentation, but the registers from the 8-bit capable controller
> are, see page 76 and following from the S805 datasheet: [0])

Okay, I am starting to understand. :-)

The spec in section 13 describes a controller supporting "MMC/SD/SDIO"
cards. However, there is yet another controller which @subject series
implement support for.
You don't happen to have a public datasheet for this controller as
well? If not, never mind.

Then, meson actually have three different MMC/SD/SDIO controllers, the
one in the S805 spec, the one supported by the recently up-streamed
meson-gx-mmc.c driver - and the one being supported in this series.
Right? And all of them are completely separate and non-compatible?

>
>>> - add support for the internal "mux" in this MMC controller (which allows
>>>   connecting up to three devices to the the controller - at the cost of
>>>   performance though since the controller can only process one request at
>>>   a time). The driver registers a new device for each sub-node, which is
>>>   then fed into the MMC framework to allow per-slot configuration using
>>>   devicetree (see the example in the documentation)
>>
>> Unless there really is deployment for more than one slot on some
>> boards/SoCs, I would strongly suggest to *not* implement this.
>>
>> Simply because of overhead and introduced complexity to the driver.
> actually there are lots of devices out there which need to use two slots:
> as mentioned above the S805 (Meson8b) SoC has two MMC controllers:
> - one which is typically used for the SD card and the SDIO wireless
> interface (the driver from this series handles this)
> - the other one is typically connects to eMMC flash (as it supports
> 8-bit bus width - this controller is not related to the driver from
> this series)

Okay.

>
> there are boards out there which use NAND flash instead of eMMC, but
> the majority of the consumer devices (based on Amlogic SoCs) out there
> uses eMMC flash.
> we (unfortunately) have to support the internal mux since there are
> three MMC devices (SD card, SDIO wifi and eMMC) but only two
> controllers.

I see.

>
> I agree with you that it adds extra complexity to the driver. I tried
> to keep it as simple as possible - but I think we cannot remove it
> (without "losing" access to one MMC devices on most boards)

Of course.

>
>>> - use the common clock framework internally for managing the MMC clock
>>>   (there is a fixed-factor clock in the controller which takes clk81 as
>>>    input and divides it's clock by two and a divider clock which takes
>>>    the result from the fixed-factor clock as input)
>>> - support the regulators provided by the MMC framework
>>> - support for GPIO-based card-detection and read-only-detection through
>>>   the MMC framework
>>> - use of the <linux/bitfield.h> FIELD_PREP and FIELD_GET macros where it
>>>   make sense (and thus the code easier to read)
>>> - re-worked locking (based on the locking in dw_mmc as that also provides
>>>   multiple "MMC slots")
>>
>> Lots of changes!
>>
>> Before even start to review (or someone else), you really need to make
>> this review-able.
>>
>> So, please, one change per patch - and make sure to write good
>> changelogs. Then I can start to review.
> from the mainline tree's perspective this is a new driver:

Yes, I get it now.

I first thought this series was related to the recently up-streamed
meson-gx-mmc.c driver. Apologize for my ignorance.

> Carlo (the original author) initially sent this driver for review more
> than 14 months ago. unfortunately it was never merged since you
> spotted some issues while reviewing that code, see [1].

Yes, I recall that now.

> I would have sent smaller patches for a driver which is already in mainline.

Right.

>
> I also wanted to avoid extra complexity for the internal mux if it was
> added later on (if we want to avoid breaking devicetree backwards
> compatibility then the driver would have to support both: parsing from
> the mmc node directly and parsing child "slot" nodes). we won't have
> to change the DT bindings when the first version that will be
> mainlined already has support for the mux
>
> please let me know if there's anything I can do to make the code
> easier to review.

No worries, let me have a look as is!

[...]

Kind regards
Uffe