linux-kernel - Re: mm/hmm: a simple question regarding devm_request_mem

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Message-ID: <20180322002352.GA12673@Asurada-Nvidia>
Date:   Wed, 21 Mar 2018 17:23:55 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: mm/hmm: a simple question regarding devm_request_mem_region()

On Wed, Mar 21, 2018 at 06:56:32PM -0400, Jerome Glisse wrote:
> On Wed, Mar 21, 2018 at 03:23:57PM -0700, Nicolin Chen wrote:
> > Hello Jerome,
> > 
> > I started to looking at the mm/hmm code and having a question at the
> > devm_request_mem_region() call in the hmm_devmem_add() implementation:
> > 
> > >	addr = min((unsigned long)iomem_resource.end,
> > >		   (1UL << MAX_PHYSMEM_BITS) - 1);
> > 
> > The main question is here as I am a bit confused by this addr. The code
> > is trying to get an addr from the end of memory space. However, I have
> > tried on an ARM64 platform where ioport_resource.end is -1, so it takes
> > "(1UL << MAX_PHYSMEM_BITS) - 1" as the addr base, while this addr is way
> > beyond the actual main memory size that's available on my board. Is HMM
> > supposed to get an memory region like this? Would it be possible for you
> > to give some hint to help me understand it?
> 
> What are you trying to do ? hmm_devmem_add() is use either for device
> private memory or device public memory. Device private memory is memory
> that is not accessible by the CPU, the code you are pointing to is for
> that case where i try to find a range of physical address not currently
> use (memory not being accessible means that there is not any valid
> physical address reserved for it). On x86 MAX_PHYSMEM_BITS is defined
> to something that make sense, but as it is often the case for those
> define, it seems that arm define an unreal value. My advice fix the
> definition for ARM iirc it depends on the SOC dunno if you can know
> that at build time. You can probably know the biggest one at build time
> (1 << 47 or something like that).
> 
> But this all assume that you have a device with its own memory that is
> not accessible from the CPU. Which is very uncommon on ARM, only case
> i know of is regular PCIE GPU on a ARM system with PCIE.

I am testing with drivers/char/hmm_dmirror.c from your git repository.

The addr I got (before "- size") is actually 0x7fffffffff, so equally
(1 << 40).

So from your reply, it seems to me that HMM is supposed to request a
region like it.

Thanks
Nicolin