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SubjectRe: [PATCH] mmc: dw_mmc: use resource_size_t to store physical address
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On Fri, Nov 13, 2015 at 11:35 AM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Friday 13 November 2015 03:10:13 Andy Shevchenko wrote:
>> On Thu, Nov 12, 2015 at 4:14 PM, Arnd Bergmann <arnd@arndb.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.

DMA operates with address space covered by dma_addr_t, if you use
phys_addr_t you may get address out of DMA boundaries. This is should
be done in hardware / firmware / platform representation.
So, I don't see any reason not to use dma_addr_t here.

>
> Arnd



--
With Best Regards,
Andy Shevchenko


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