[PATCH] simplebus child device probe order control via FDT (motivated by BeagleBone Black)

[Warner Losh]

On Mar 5, 2014, at 11:55 AM, Patrick Kelsey <kelsey at ieee.org> wrote:

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> 
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> On Wed, Mar 5, 2014 at 8:31 AM, Warner Losh <imp at bsdimp.com> wrote:
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> On Mar 4, 2014, at 11:53 PM, Patrick Kelsey <kelsey at ieee.org> wrote:
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> >
> >
> >
> > On Tue, Mar 4, 2014 at 8:21 AM, Ian Lepore <ian at freebsd.org> wrote:
> >
> > There's a standard property for mmc/sd devices, "non-removable" whose
> > presence denotes things like soldered-on eMMC parts.  Only one of our
> > many sdhci drivers supports it right now (imx).  In general the core
> > part of the driver (dev/sdhci) doesn't have good support for
> > insert/remove/presence detection that's handled off to the side (whether
> > it's non-removable or a gpio pin).
> >
> > That alone doesn't solve the wider problem, though, which I think breaks
> > down into two pieces.  Let's say you've got two slots, call them left
> > and right.  You end up with these two problems...
> >
> >       * Sometimes the right slot is mmcsd0, but it turns into mmcsd1 if
> >         there's a card in the left slot when I boot; I don't want it to
> >         change.
> >
> > And not just a boot-time issue, of course.  If you were to remove those two cards and then reinsert them in the opposite time-order, their device names would swap.
> >
> >       * I want the slot on the left to be named '1' and the right to be
> >         '0'.
> >
> > The first problem is easily solved without impacting dts in any way.  We
> > just wire down the relationship controllerN -> mmcN -> mmcsdN.  This is
> > exactly equivelent to the old ATA_STATIC_ID option in its effect -- you
> > don't get to choose the names, but you know they won't change based on
> > which devices are present.  It could be controlled with a tunable.
> >
> > It's harder to envision the fix for the second part without adding an
> > ad-hoc property for the devices.  Even with a property I'm not sure how
> > easy it would be.
> >
> > We should be able to assign a geographic address (controllerX:slotY) to each mmcsd device in a given system (let's ignore for now the theoretical possibility of multiple cards on one bus).  The 'controllerX' part of the address could be the controller's pathname from the devicetree, or an index assigned by mmcbr machinery at attach time.  The "slotY" part of the address is assigned by the specific controller device driver using some internally-determined fixed mapping between the assigned values and its physical slots.  This geographic address could be used to create an additional set of /dev entries with stable names.  There could be a mechanism for user-configurable aliases for the geographical addresses.
> 
> There’s already a chance to run a script when a device is attached that can create /dev/slot0 or /dev/slot1 that has geographical information available to it. People use ddvd for this in the USB world all the time to make sure that tty devices get a symlink based on their location or serial number.
> 
> Why is mmc so different it needs its own mechanism?
> 
> I'm laying out my view of the information that would be needed and the types of actions that would have to be taken based on that information to solve the issue.  I'm not saying devd can't be the piece that is used to implement the actions (indeed, I noted devd as a potential building block for a solution in my initial response).  I'm also not saying mmc needs its own mechanism, I'm saying it needs /a/ mechanism, and so far there still seems to be something missing (because it's not there, or I'm still ignorant of it).
> 
> What seems to be missing is geographical addressing for mmc devices.
> 
> I think what you might be saying is that the existing mmcsd add/remove code could be augmented to send devctl notifications, along the lines of:
> 
> devctl_notify("MMC", "DEVICE", "ATTACH"|"DETACH", "... controller=<controller_device_name_or_better_yet_devicetree_path> slot=<slot_number> rca=<rca>")
> 
> and then I and the fine author of devctl and devd would be pleased.

MMC doesn’t need anything special here. That’s already present. Looking at the device tree we see on one of the platforms that’s handy for me to access:

    at91_mci0
      mmc0
        mmcsd0 at rca=0xb368

So you know that mmcsd0 is on mmc0 bus attached to at91_mci0, which is ultimately the FDT node where things came from. There’s not a user-defined slot associated with this (and we should have a SLOT A vs SLOT B as a location info for this platform, because we can have two cards on the one bus in the MMC case), true, but for your use case I don’t think that you need it. We should be attaching the host controller regardless of whether the or not there’s a card in there, so it will be fixed. While some additional information would be useful to publish, I don’t think your use case requires it…

Warner