RE: [PATCH 003/117] Staging: hv: Add struct hv_vmbus_device_id tomod_devicetable.h

[KY Srinivasan]

> -----Original Message-----
> From: Greg KH [mailto:greg@xxxxxxxxx]
> Sent: Wednesday, August 24, 2011 4:12 PM
> To: KY Srinivasan
> Cc: gregkh@xxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> devel@xxxxxxxxxxxxxxxxxxxxxx; virtualization@xxxxxxxxxxxxxx; Haiyang Zhang
> Subject: Re: [PATCH 003/117] Staging: hv: Add struct hv_vmbus_device_id to
> mod_devicetable.h
> 
> On Wed, Aug 24, 2011 at 04:46:11PM +0000, KY Srinivasan wrote:
> >
> >
> > > -----Original Message-----
> > > From: Greg KH [mailto:greg@xxxxxxxxx]
> > > Sent: Tuesday, August 23, 2011 10:59 PM
> > > To: KY Srinivasan
> > > Cc: gregkh@xxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > > devel@xxxxxxxxxxxxxxxxxxxxxx; virtualization@xxxxxxxxxxxxxx; Haiyang Zhang
> > > Subject: Re: [PATCH 003/117] Staging: hv: Add struct hv_vmbus_device_id to
> > > mod_devicetable.h
> > >
> > > On Wed, Aug 24, 2011 at 12:44:38AM +0000, KY Srinivasan wrote:
> > > >
> > > >
> > > > > -----Original Message-----
> > > > > From: Greg KH [mailto:greg@xxxxxxxxx]
> > > > > Sent: Tuesday, August 23, 2011 6:42 PM
> > > > > To: KY Srinivasan
> > > > > Cc: gregkh@xxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > > > > devel@xxxxxxxxxxxxxxxxxxxxxx; virtualization@xxxxxxxxxxxxxx; Haiyang Zhang
> > > > > Subject: Re: [PATCH 003/117] Staging: hv: Add struct hv_vmbus_device_id
> to
> > > > > mod_devicetable.h
> > > > >
> > > > > On Fri, Jul 15, 2011 at 10:45:51AM -0700, K. Y. Srinivasan wrote:
> > > > > > In preparation for implementing vmbus aliases for auto-loading
> > > > > > Hyper-V drivers, define vmbus specific device ID.
> > > > > >
> > > > > > Signed-off-by: K. Y. Srinivasan <kys@xxxxxxxxxxxxx>
> > > > > > Signed-off-by: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
> > > > > > ---
> > > > > >  include/linux/mod_devicetable.h |    7 +++++++
> > > > > >  1 files changed, 7 insertions(+), 0 deletions(-)
> > > > > >
> > > > > > diff --git a/include/linux/mod_devicetable.h
> > > > > b/include/linux/mod_devicetable.h
> > > > > > index ae28e93..5a12377 100644
> > > > > > --- a/include/linux/mod_devicetable.h
> > > > > > +++ b/include/linux/mod_devicetable.h
> > > > > > @@ -405,6 +405,13 @@ struct virtio_device_id {
> > > > > >  };
> > > > > >  #define VIRTIO_DEV_ANY_ID	0xffffffff
> > > > > >
> > > > > > +/*
> > > > > > + * For Hyper-V devices we use the device guid as the id.
> > > > > > + */
> > > > > > +struct hv_vmbus_device_id {
> > > > > > +	__u8 guid[16];
> > > > > > +};
> > > > >
> > > > > Why do you not need a driver_data pointer here?  Are you sure you aren't
> > > > > ever going to need it in the future?  Hint, I think you will...
> > > >
> > > > I am not sure I am following you here; the guid is the device ID and it is
> > > > guaranteed to remain the same. What is the driver _data pointer here
> > > > you are referring to here.  While some device id have the _data pointer,
> > > > there are others that don't - for instance struct virtio_device_id. In
> > > > our case, I am not sure how I would use this private pointer.
> > >
> > > You use it like all other drivers use it, only if needed.
> >
> > Fair enough; the point is I am not sure how I would use it.
> >
> > >
> > > Hint, I think you need to use it in your hv_utils driver, it would
> > > reduce the size of your code and simplify your logic.
> >
> > Could you expand on this. Currently the util driver handles a bunch
> > services that have their own guids - and these have been included
> > in the idtable. How would having the pointer simplify this code.
> 
> It would allow you, in your probe function, to do something different
> depending on the guid that the probe function was matching on.  So you
> would not have to check the guid again to do that, just use the data
> pointed in that void pointer and away you go.
> 
> As an example, look at drivers/usb/class/cdc-acm.c, the acm_ids[]
> variable which uses the driver_info field to contain a quirk for the
> device.

Ok; this makes sense. But I currently don't have any quirks to support!
The util driver is not even a driver in the true sense. I made it a driver and
added the probe function just to support auto-loading with the vmbus ID space
that I am trying to implement here - the probe function does nothing.

So, if it is ok with you, I will not add driver_data pointer since I will not be
using it.
 
> 
> > I looked at the usage of this in PCI and it appears to be for supporting
> > dynamic  IDs for existing drivers.
> 
> No, that's exactly wrong.  dynamic ids play havoc with this pointer,
> making some drivers not be able to handle dynamic ids because they rely
> on it for some driver-specific information to be passed in it, which
> dynamic ids can not handle.
> 
> Oh, have you remembered to turn off dynamic ids for these devices?  Or
> do you support them properly?

I don't support dynamic IDs. What would I need to do to turn it off.

Regards,

K. Y
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