miibus + USB = problem

[Hans Petter Selasky]

On Thursday 03 August 2006 04:25, Pyun YongHyeon wrote:
> On Wed, Aug 02, 2006 at 02:37:55PM +0200, Hans Petter Selasky wrote:
>  > Hi,
>  >
>  > I am currently in the progress of porting "if_aue.c" to my new USB API,
>  > and have hit a problem that needs to be solved. The problem is that the
>  > USB system sleeps under "miibus_xxx". Questions are:
>  >
>  > Is this allowed?
>  >
>  > What locks are held when these functions are called ?
>  >
>  > Reference:
>  >
>  >  /* MII interface */
>  >  DEVMETHOD(miibus_readreg, aue_miibus_readreg),
>  >  DEVMETHOD(miibus_writereg, aue_miibus_writereg),
>  >  DEVMETHOD(miibus_statchg, aue_miibus_statchg),
>
> AFAIK there is no locks held when MII layer calls above methods but
> it _is_ called with a driver lock from its ioctl handler.

Ok.

> It seems that aue(4) needs to access its register space whilst
> serving above MII methods. 

...

> But I think sleeping is not a good way in the MII methods as it would
> confuse MII layers. See below.
>

Yes, "mii_tick()" can suddenly be executed while "mii_mediachg()" is being 
executed.

But can I assume that everytime something inside the "mii" layer is called, it 
goes through my driver, "if_aue".

From what I can see, there are only three entry points into the MII layer:

mii_mediachg(mii);
mii_pollstat(mii);
mii_tick(mii);

The idea is to call these from a separate thread, that can sleep. But then 
comes the next question:

Can I safely "kthread_exit" from the "miibus_xxx()" functions ? I can probably 
find the answer by looking at the sourcecode, but maybe someone already 
knows?

/* MII interface */
DEVMETHOD(miibus_readreg, aue_miibus_readreg),
DEVMETHOD(miibus_writereg, aue_miibus_writereg),
DEVMETHOD(miibus_statchg, aue_miibus_statchg),

>
>
> I think you can immediately return from register read/write routines
> without DELAY() if you know the hardware was gone.
>
>  > 2) pre-read all read registers regularly. How can I do this with
>  > "miibus"?
>
> Because you have a aue_tick() which is called every hz you can cache
> several registers regularly. Your MII methods can copy the value
> without accessing registers with proper locks. However, this may
> confuse MII layers because it needs successive register accesses to
> read/write media related settings and that defeats use of cached
> contents of the registers.

I just have to cache the result:

        ifmr->ifm_active = mii->mii_media_active;
        ifmr->ifm_status = mii->mii_media_status;

Thanks for any comments.

--HPS