Re: [PATCH] mmc: dw_mmc: use resource_size_t to store physical address

[Jaehoon Chung <jh80.chung@samsung.com>]

Dear, Arnd.

On 11/13/2015 06:35 PM, Arnd Bergmann 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.

why isn't the patch applied on mainline yet? :)

Best Regards,
Jaehoon Chung

> 
>>>         /* 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
>