[PATCH v2] ARM: shmobile: silk: add SDHI1 DT support

[Magnus Damm]

Hi Sergei,

On Wed, Sep 23, 2015 at 8:19 AM, Sergei Shtylyov
<sergei.shtylyov at cogentembedded.com> wrote:
> Hello.
>
>
> On 09/18/2015 05:56 AM, Magnus Damm wrote:
>
>>>>>>>>> Define the SILK board dependent part of the SDHI1 (connected to
>>>>>>>>> micro-SD slot)
>>>>>>>>> device nodes along with the necessary voltage regulators.
>>>>>>>>>
>>>>>>>>> Based on the original patch by Vladimir Barinov
>>>>>>>>> <vladimir.barinov at cogentembedded.com>.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Sergei Shtylyov <sergei.shtylyov at cogentembedded.com>
>>>>>>>>>
>>>>>>>>> ---
>>>>>>>>> This patch is against 'renesas-devel-20150810-v4.2-rc6' tag of
>>>>>>>>> Simon Horman's
>>>>>>>>> 'renesas.git' repo plus the R8A7794/SILK QSPI patches just
>>>>>>>>> re-posted. It needs
>>>>>>>>> the R8A7794 GPIO patches in order to compile.
>>>>>>>>>
>>>>>>>>> Changes in version 2:
>>>>>>>>> - removed not working SDHI0 stuff, renamed the patch;
>>>>>>>>> - replaced SDHI1's "wp-gpios" property with "disable-wp".
>>>>>>>>
>>>>>>>>
>>>>>>>> I am wondering if you could explain the motivation for the
>>>>>>>> "disable-wp"
>>>>>>>> update
>>>>>>>
>>>>>>>
>>>>>>>     Please see the comment in mmc_sd_get_ro().
>>>>>>>
>>>>>>>> and weather it is appropriate for other r8a779* dts files.
>>>>>>>
>>>>>>>
>>>>>>>     In case of micro-SD slots, yes.
>>>>>>
>>>>>>
>>>>>>     The MMC binding document says it should only be specified if the
>>>>>> controller has WP detection logic. We, so far, seem to have been
>>>>>> replying on
>>>>>> the GPIOs despite this logic is present (according to the R-Car gen2
>>>>>> SDHI
>>>>>> manuals I have). The driver will first call mmc_gpio_get_ro() and when
>>>>>> that
>>>>>> fails due to "wp-gpios" not being specified, it proceeds to reading
>>>>>> the
>>>>>> register but that is forbidden by TMIO_MMC_WRPROTECT_DISABLE flag set
>>>>>> for
>>>>>> the R-Car gen1/2 chips, so 0 is always returned from
>>>>>> tmio_mmc_get_ro().
>>>>>> There seems to be no point in going that far (and doing the runtime PM
>>>>>> dances) --
>>>>
>>>>
>>>>     Alternatively, the driver could be fixed to check the flag before
>>>> the RPM
>>>> call unlike what it does now.
>>>
>>>
>>> If the driver can be updated to do the right thing then that seems
>>> preferable to me. If so would it be the case that the presence of the
>>> "disable-wp" property would not have any run-time effect?
>>>>
>>>>
>>>>>> and MMC_CAP2_NO_WRITE_PROTECT (set when "disable-wp" is specified)
>>>>>> prohibits
>>>>>> doing that...
>>>>>
>>>>>
>>>>> That sounds reasonable to me.
>>>>
>>>>
>>>>     I'm still wondering why TMIO_MMC_WRPROTECT_DISABLE is set for the
>>>> R-Car
>>>> SoCs. Morimoto-san, any comments? Your change logs are too terse. :-)
>>>
>>>
>>> I will follow up on this.
>
>
> [...]
>>
>> Now what is the issue that you guys are having?
>
>
>    My main issue is that I don't understand why checking the write protect
> bit is always prohibited for the R-Car SoCs (by setting
> TMIO_MMC_WRPROTECT_DISABLE), i.e. it can only be read via GPIO (though that
> GPIO is just an alias of the WP signal).

I believe the reason is that we decided to keep it simple - so we
preferred to use GPIO instead of native SDHI signals. So GPIO WP
instead of the not-always-present SDHI WP signal. Historically CD and
WP may on some boards be routed on different pins than the SDHI CD and
WP lines, and if we support both GPIO and native SDHI signals we need
to handle both cases. With GPIO there is only one case to handle. And
it is not exactly hot path to handle WP and CD so the overhead must be
minimal.

Historically there have been were cases with people submitting board
support patches and getting the WP signal wrong. It seems difficult
for people to understand that the WP signal is missing in case of
micro-SD sockets. With the incorrect WP signal the cards would end up
being read only. Combine that with lack of board schematics and it all
turns into a big mess.

The on-chip SoC SDHI devices in DT and the driver on R-Car Gen2
assumes no WP and CD signals by default. It is up to each board to
opt-in to add the GPIOs for WP and CD. It is very simple and should
make it possible to power down the SDHI instances if no cards are
inserted and let the GPIO IRQ wake up things for us.

I still don't understand what the real problem is though...

Thanks,

/ magnus