To: "Caitlin Bestler" <caitlinb@broadcom.com>
Cc: "Grant Grundler" <iod00d@hp.com>, openib-general@openib.org,
       linux-kernel@vger.kernel.org,
       "Segher Boessenkool" <segher@kernel.crashing.org>
Subject: Re: [openib-general] Re: [PATCH 21 of 53] ipath - use phys_to_virt instead of bus_to_virt
References: <54AD0F12E08D1541B826BE97C98F99F149F34B@NT-SJCA-0751.brcm.ad.broadcom.com>
From: Roland Dreier <rdreier@cisco.com>
Date: Mon, 15 May 2006 16:50:11 -0700
In-Reply-To: <54AD0F12E08D1541B826BE97C98F99F149F34B@NT-SJCA-0751.brcm.ad.broadcom.com> (Caitlin Bestler's message of "Mon, 15 May 2006 16:40:35 -0700")
Message-ID: <adabqtyrgp8.fsf@cisco.com>
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    Caitlin> True, but how does that constrain the local interfaces by
    Caitlin> which the driver is informed of the set of pages that
    Caitlin> back a given memory region? The driver must still
    Caitlin> ultimately provide dma accessible addresses to the
    Caitlin> device. RDMA just changes the timing of the steps, albeit
    Caitlin> radically, but not what the steps are.

It's only a problem for "reserved L_Key" types of things, where the
device is supposed to just use the address given in a work request
without translating it.  No translation means that work requests have
to contain "bus addresses" -- addresses that are what the device would
put on the bus to access memory.  But if a device needs to simulate
DMA in software, then it really needs a kernel virtual address, not a
bus address.  But it's pretty ugly to have to put that knowledge in
every consumer.

 - R.
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