On 21.01.2014 19:34, Tomasz Figa wrote:
Hi,
On 20.01.2014 04:56, Olof Johansson wrote:
This patch enables support for power-on sequencing of SDIO peripherals
through DT.
In general, it's quite common that wifi modules and other similar
peripherals have several signals in addition to the SDIO interface that
needs wiggling before the module will power on. It's common to have a
reference clock, one or several power rails and one or several lines
for reset/enable type functions.
The binding as written today introduces a number of reset gpios,
a regulator and a clock specifier. The code will handle up to 2 gpio
reset lines, but it's trivial to increase to more than that if needed
at some point.
Implementation-wise, the MMC core has been changed to handle this during
host power up, before the host interface is powered on. I have not yet
implemented the power-down side, I wanted people to have a chance for
reporting back w.r.t. issues (or comments on the bindings) first.
I have not tested the regulator portion, since the system and module
I'm working on doesn't need one (Samsung Chromebook with Marvell
8797-based wifi). Testing of those portions (and reporting back) would
be appreciated.
While I fully agree that this is an important problem that needs to be
solved, I really don't think this is the right way, because:
a) power-up sequence is really specific to the MMC device and often it's
not simply a matter of switching on one regulator or one clock, e.g.
specific time constraints need to be met.
b) you can have WLAN chips in which SDIO is just one of the options to
use as host interface, which may be also HSIC, I2C or UART. Really. See
[1].
c) this is leaking device specific details to generic host code, which
isn't really elegant.
Now, to make this a bit more constructive, [2] is a solution that I came
up with (not perfect either), which simply adds a separate platform
device for the low level part of the chip. I believe this is a better
solution because:
a) you can often see such WLAN/BT combo chip as a set of separate
devices, e.g. SDIO WLAN, UART BT and a simple PMIC or management IC,
which provides power/reset control, out of band signalling and etc. for
the first two, so it isn't that bad to have a separate device node for
the last one,
b) you have full freedom of defining your DT binding with whatever data
you need, any number of clocks, regulators, GPIOs and even out of band
interrupts (IMHO the most important one).
c) you can implement power-on, power-off sequences as needed for your
particular device,
d) you have full separation of device-specific data from MMC core (or
any other subsystem simply used as a way to perform I/O to the chip).
Now what's missing there is a way to signal the MMC core or any other
transport that a device showed up and the controller should be woken up
out of standby and scan of the bus initialized. This could be done by
explicitly specifying the device as a subnode of the
MMC/UART/USB(HSIC)/I2C or whatever with a link (phandle) to the power
controller of the chip or the other way around - a link to the
MMC/UART/... controller from the power controller node.
I've looked a bit around MMC core code and got some basic idea how
things look. I will definitely need some guidance, or at least some
opinions, from MMC guys, as some MMC core changes are unavoidable.
Now, the device-specific code is not really an issue, existing drivers
usually already have their ways of powering the chips on and off, based
on platform data. Everything needed here is to retrieve needed resources
(GPIOs, clocks, regulators) using DT, which should be trivial.
The worse part is the interaction between MMC and power controller
driver (the platform driver part of WLAN driver, if you look at brcmfmac
as an example). I believe that we need following things:
a) A way to tell the MMC controller that there is no card detection
mechanism available on given slot and it also should not be polling the
slot to check card presence. Something like a "manual card detect" that
would be triggered by another kernel entity that controls whether the
MMC device is present (i.e. WLAN driver). We already have "broken-cd"
property, but it only implies the former, wasting time on needless polling.
b) A mechanism to bind the power controller to used MMC slot. Something
like "mmc-bus = <&mmc2>;" property in device node of the power
controller and a function like of_find_mmc_controller_by_node(), which
would be an MMC counterpart of I2C's of_find_i2c_adapter_by_node(). To
avoid races, it should probably take a reference on MMC host that would
have to be dropped explicitly whenever it is not needed anymore.
c) A method to notify the MMC subsystem that card presence has changed.
We already have something like this in drivers/mmc/core/slot-gpio.c, but
used for a simple GPIO-based card detection. If the main part of
mmc_gpio_cd_irqt() could be turned into an exported helper, e.g.
mmc_force_card_detect(host) then basically we would have everything needed.
Unfortunately, I don't have more time left for today to create patches
and test them, so for now, I'd like to hear opinion of MMC maintainers
about this approach. Do you find this acceptable?
By the way, it seems like slot-gpio.c could replace a lot of custom GPIO
card detection code used in MMC host drivers, e.g. sdhci-s3c. Is there
any reason why it couldn't?
Best regards,
Tomasz
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