linux-kernel - Re: Juju

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Message-ID: <45BA5BBE.7060709@redhat.com>
Date:	Fri, 26 Jan 2007 14:51:26 -0500
From:	Kristian Høgsberg <krh@...hat.com>
To:	Greg KH <greg@...ah.com>
CC:	Pete Zaitcev <zaitcev@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: Juju

Greg KH wrote:
> On Thu, Jan 25, 2007 at 03:38:24PM -0800, Pete Zaitcev wrote:
>> On Thu, 25 Jan 2007 16:18:35 -0500, Kristian H??gsberg <krh@...hat.com> wrote:
>>
>>>> I see that ORBs are always allocated with a call (like SKB) and not
>>>> embedded into drivers (like URBs). It's great, keep it up. Also,
>>>> never allow drivers to pass DMA-mapped buffers into fw_send_request
>>>> and friends. We made both of these mistakes in USB, and it hurts.
>>> Oh, the ORBs are SBP-2 specific data structures, struct fw_transaction is 
>>> probably what corresponds to USB URBs.  This struct is defined in 
>>> fw-transaction.h and is available for embedding into other structs, such as 
>>> struct sbp2_orb in fw-sbp2.  Is that what you're suggesting against, and what 
>>> are the problems with this approach?
>> Fortunately we do not care about out-of-tree drivers, which are most
>> affected, you may even call it a feature ^_^. My main problem is,
>> we can't refcount URBs, so usbmon can't tap them and must copy.
> 
> urbs are reference counted, it's just that not all drivers who create
> them use them that way :(
> 
> Perhaps you can inforce this in the new codebase...

It's a small change to make the fw_transaction struct opaque and ref-counted, 
and it's definitely still doable.  But the nice thing about embedding the 
struct is that you have one memory allocation failure path less to worry 
about.  And I haven't yet, and don't expect to see a use case that will need 
ref-counted struct fw_transaction, the ownership is always clearly defined. 
But I can go either way on this and if there is a good reason to ref count 
them it's a pretty small change.

cheers,
Kristian
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