linux-kernel - Re: [PATCH] mmc: dw_mmc: use resource_size

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Date:	Fri, 13 Nov 2015 10:35:16 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Andy Shevchenko <andy.shevchenko@...il.com>
Cc:	Ulf Hansson <ulf.hansson@...aro.org>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] mmc: dw_mmc: use resource_size_t to store physical address

On Friday 13 November 2015 03:10:13 Andy Shevchenko wrote:
> On Thu, Nov 12, 2015 at 4:14 PM, Arnd Bergmann <arnd@...db.de> wrote:
> > The dw_mmc driver stores the physical address of the MMIO registers
> > in a pointer, which requires the use of type casts, and is actually
> > broken if anyone ever has this device on a 32-bit SoC in registers
> > above 4GB. Gcc warns about this possibility when the driver is built
> > with ARM LPAE enabled:
> 
> > -       host->phy_regs = (void *)(regs->start);
> > +       host->phy_regs = regs->start;
> 
> >         /* Set external dma config: burst size, burst width */
> > -       cfg.dst_addr = (dma_addr_t)(host->phy_regs + fifo_offset);
> > +       cfg.dst_addr = host->phy_regs + fifo_offset;
> 
> dst_addr is dma_addr_t?

Sort of. It doesn't really fit into any of the categories, and we actually
had a patch to change the type in the past, see
https://lkml.org/lkml/2015/7/10/167. Not sure what is going on there.

> >         /* Registers's physical base address */
> > -       void                    *phy_regs;
> > +       resource_size_t         phy_regs;
> 
> If dst_addr is dma_addr_t wouldn't be a problem when
> resource_size_t is defined as 64-bit address, and dma_addr_t as 32-bit?
> 
> Btw, for me casting to dma_addr_t looks sane.

The background here is that the address comes from a resource_size_t
that describes the MMIO register area as seen from the CPU, and that
is normally a phys_addr_t (resource_size_t is defined as being long
enough to store a phys_addr_t or various other things depending on
resource->flags).

dma_addr_t strictly speaking refers to a RAM location as seen by a
DMA master, and that only comes out of dma_map_*() or
dma_alloc_coherent().

The DMA engine wants something else here, which is an MMIO register
address as seen by a DMA master, and we don't have a separate typedef
for that. Almost universally all of resource_size_t, phys_addr_t and
dma_addr_t are the same type, and if we ever get a platform that
wants something other than a phys_addr_t to put into cfg.dst_addr,
we are in deep trouble.

	Arnd
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