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#1: virtual -->physical --> virtual address

Posted on 2011-10-15 09:01:08 by ratheesh kannoth

{
virtual_addr1 = kmalloc(0x20, GFP_USER);
phy_addr1 = __pa(virtual_addr1 );
virtual_addr2 = ioremap(phy_addr1, 0x20 );

}


I noticed that virtual_addr1 is not equal to virtual_addr2. And if i
dereference virtual_addr2 ( *virtual_addr2 ),
kernel crash. What is wrong here ?

-Ratheesh

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#2: Re: virtual -->physical --> virtual address

Posted on 2011-10-15 19:57:35 by Mirko Banchi

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Il giorno 15/ott/2011, alle ore 09.01, ratheesh kannoth ha scritto:

> {
> virtual_addr1 = kmalloc(0x20, GFP_USER);
> phy_addr1 = __pa(virtual_addr1 );
> virtual_addr2 = ioremap(phy_addr1, 0x20 );
>
> }
>
>
> I noticed that virtual_addr1 is not equal to virtual_addr2. And if i
> dereference virtual_addr2 ( *virtual_addr2 ),
> kernel crash. What is wrong here ?
>

Obviously virtual_addr1 and virtual_addr2 are not equals! kmalloc returns a logical address (in fact you can simply calc the relative physical address with macro __pa) and ioremap returns a virtual address in the range [VMALLOC_START - VMALLOC_END]. However usually ioremap is used to deal with iomem and you shouldn't dereference the returned pointer directly: you should use instead ioread{8,16,32} or iowrite{8,16,32} family functions in order to make code portable. However i try to write a simple LKM with your code and i can dereference the pointer returned by ioremap correctly. Can you post some detailed code?

Mirko

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Mirko Banchi

e-mail: mk.banchi-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
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