linux-kernel - Re: [PATCH 1/6] pmem: add force casts to avoid _

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Date:	Fri, 29 May 2015 05:39:44 -0600
From:	Ross Zwisler <zwisler@...il.com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Linux ACPI <linux-acpi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH 1/6] pmem: add force casts to avoid __iomem annotation

On Thu, 2015-05-28 at 15:47 -0700, Dan Williams wrote:
> On Thu, May 28, 2015 at 3:35 PM, Ross Zwisler
> <ross.zwisler@...ux.intel.com> wrote:
> > Even though we use ioremap_nocache() to map our persistent memory 
> > in the
> > pmem driver, the memory we are mapping behaves like normal memory 
> > and
> > not I/O memory in that it can be accessed using regular memcpy()
> > operations and doesn't need to go through memcpy_toio() and
> > memcpy_fromio().  Force casting the pointers received from
> > ioremap_nocache() and given to iounmap() gives us the correct 
> > behavior
> > and makes sparse happy.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> > Cc: Dan Williams <dan.j.williams@...el.com>
> > Cc: linux-nvdimm@...ts.01.org
> > ---
> >  drivers/block/nd/pmem.c | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/block/nd/pmem.c b/drivers/block/nd/pmem.c
> > index 5e8c9c629f22..a8712e41e7f5 100644
> > --- a/drivers/block/nd/pmem.c
> > +++ b/drivers/block/nd/pmem.c
> > @@ -163,7 +163,8 @@ static struct pmem_device *pmem_alloc(struct 
> > device *dev, struct resource *res,
> >          * of the CPU caches in case of a crash.
> >          */
> >         err = -ENOMEM;
> > -       pmem->virt_addr = ioremap_nocache(pmem->phys_addr, pmem
> > ->size);
> > +       pmem->virt_addr = (__force void *)ioremap_nocache(pmem
> > ->phys_addr,
> > +                       pmem->size);
> 
> I think I'd rather see casting when ->virt_addr is used (the
> __io_virt() helper can be used to make this a tad cleaner), or 
> provide
> ioremap apis that don't attach __iomem to their return value. 
>  Because
> in this and other cases ioremap() is being on non "i/o" memory.

The reason that I thought this was cleaner was that now when you look
at the pmem->virt_addr definition it is just a clean void* with no
annotations.  This correctly describes the memory to the user (it's
usable as regular memory, it's in the kernel address space, etc.).

Having the pointer itself annotated with __iomem feels weird to me
because a random well meaning user could incorrectly try to use it as
I/O memory.
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