[RFC] smsc911x device tree binding questions

[Lorenzo Pieralisi]

> -----Original Message-----
> From: Steve.Glendinning at smsc.com [mailto:Steve.Glendinning at smsc.com]
> Sent: 29 August 2010 13:00
> To: Grant Likely; Lorenzo Pieralisi
> Cc: Catalin Marinas; David Miller; devicetree-discuss at lists.ozlabs.org;
> glikely at secretlab.ca; Ian.Saturley at smsc.com
> Subject: Re: [RFC] smsc911x device tree binding questions
> 
> Hi Lorenzo,
> 
> > > - the SMSC911x host bus size can be retrieved from the chip idrev (for
> > > chip idrev having just 16-bit bus IF) or for controllers like 9118
> that
> > > can have both (32/16), there is a D32/D16 HW strap readable from
> HW_CFG.
> > > The problem with these flags is that they define bus size from a CPU
> > > perspective if I got it right, in particular if the CPU can access
> > > the SMSC with 32-bit load/store to access 32 bit registers or
> > > have to resort to two "locked" 16-bit accesses to ensure
> > > atomicity. I think it is possible on some architectures to use a
> 32-bit
> > > load/store even if the smsc911x IF is 16-bit (e.g. bus downsize
> converters,
> > > which ensure 16-bit load/store locking).
> > > It depends on the bus interconnect architecture. I think we
> > > need a property to define this, thoughts much appreciated.
> 
> Absolutely right - this flag defines whether the DRIVER has to use 16
> or 32 bit accesses in order to succesfully access the chip.  If the bus
> hardware is capable of performing atomic pairs of 16-bit operations
> when we make a 32-bit read then this is very much preferred.  The 16-bit
> path is MUCH slower as it has to take a spinlock on every access to
> ensure the two reads/writes are atomic.  It's there as a last resort,
> and its use is discouraged if avoidable.
> 
> Autodetecting the bus width from register values is interesting, you'd
> have to test that you can reliably read the registers in all of the above
> modes.  It's my understanding that if the device is strapped for 16-bit
> access it expects to see atomic pairs of reads, so I guess you could make
> a pair of 32-bit reads (of something known like BYTE_TEST) and analyse
> what you get back to determine how you're connected?

Yes, we could try something like the code in smsc911x_init 
(BYTE_TEST pair of reads) but more generic to "probe" for the interconnection 
and set the flags accordingly. 

A question though(16-bit/32-bit controllers, e.g. 9118):

If the device is strapped to 16-bit host bus mode, a single read 32-bit would 
reveal us if the bus converts the load 32-bit to two load 16-bit or
not because the D[31:16] data bus lines are left in a high impedance state. 
Right ?

Maybe a pair of reads - same address -  is safer, the controller invalidate 
the read (what does it mean in terms of bytes read ?) if it samples the same 
address (load 32 not split in 16-bit loads) on consecutive reads.
If the conversion is correct the smsc returns the BYTE_TEST value otherwise 
we would notice the problem.  

I can write the code in the DT port context, but I do not have any
16-bit strapped smsc lan controller at hand, so I cannot test it, but I can post
the patch for review and testing on the list.
 
> > > * forcing internal/ext PHY
> > >
> > > #define SMSC911X_FORCE_INTERNAL_PHY             (BIT(2))
> > > #define SMSC911X_FORCE_EXTERNAL_PHY     (BIT(3))
> > >
> > > - The device tree allows to describe the mdio bus and all PHYs which
> are present
> > > on a given controller. Again for controllers having just an internal
> PHY there
> > > is nothing to force, just use the internal PHY and that's it, that's
> what
> > > the driver does. If the controller supports both EXT and internal PHY,
> > > since the internal PHY is always at address 0x1 (am I correct ?), we
> may define
> > > something like:
> > > if the tree defines a PHY @ 0x1 ("reg" property) let's use it (force
> it or at
> > > least higher priority than ext phy if both are defined), otherwise if
> the tree
> > > defines a PHY which is not @ 0x1 let's use it (force ext).
> > > If no PHY is specified in the tree, let's resort to HW probing
> (EXT_PHY_DET in
> > > HW_CFG reg).
> >
> > I'd say, if there is no phy data provided in the tree then default to
> > the internal phy.  Otherwise, use the phy pointed to by the phy-device
> > property.  It is also possible to specify a phy in the tree without an
> > address to make the kernel probe for the address (although I need to
> > double check that the support for it has actually been merged).
> >
> > > All in all, this means: just use the info from the tree, if there are
> no
> > > PHYs specified resort to HW probing for EXT PHY, if it fails use
> internal.
> > >
> > > Acceptable ?
> 
> Yes, bearing in mind that the majority of users do use the internal PHY
> this
> sounds like a sensible plan.
> 
> > > * save MAC ADDRESS
> > >
> > > #define SMSC911X_SAVE_MAC_ADDRESS               (BIT(4))
> > >
> > > -  Since we can encode the mac address in the device tree
> ("local-mac-address"
> > > property) I am not sure this flag is useful if probing from the tree
> is enabled.
> > > It is used if the chip is not fitted with an EEPROM, to save the MAC
> address
> > > set by a boot-loader before resetting the chip (at probe). Since we
> can still
> > > retrieve the MAC from the tree and reprogram it when the content of
> the
> > > registers is gone at reset, maybe there is no point in using the
> option
> > > (if probing from the tree) or at least no point in defining the flag
> in
> > > the tree.
> >
> > I'd agree, this isn't really useful in the device tree use case; but a
> > property can be added for it if it was really necessary.
> 
> Yes, this is just a workaround for a specific use case: the bootloader
> "owns" the
> MAC address configuration data and it's relatively hard to access this
> storage once
> Linux boots.

Ok, all points taken, binding and relative code in progress.